Classifications

• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
  • C08F283/00—Macromolecular compounds obtained by polymerising monomers on to polymers provided for in subclass C08G
  • C08F283/10—Macromolecular compounds obtained by polymerising monomers on to polymers provided for in subclass C08G on to polymers containing more than one epoxy radical per molecule
• • C—CHEMISTRY; METALLURGY
  • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
  • C09J—ADHESIVES; NON-MECHANICAL ASPECTS OF ADHESIVE PROCESSES IN GENERAL; ADHESIVE PROCESSES NOT PROVIDED FOR ELSEWHERE; USE OF MATERIALS AS ADHESIVES
  • C09J163/00—Adhesives based on epoxy resins; Adhesives based on derivatives of epoxy resins
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
  • C08L2666/00—Composition of polymers characterized by a further compound in the blend, being organic macromolecular compounds, natural resins, waxes or and bituminous materials, non-macromolecular organic substances, inorganic substances or characterized by their function in the composition
  • C08L2666/02—Organic macromolecular compounds, natural resins, waxes or and bituminous materials
  • C08L2666/04—Macromolecular compounds according to groups C08L7/00 - C08L49/00, or C08L55/00 - C08L57/00; Derivatives thereof
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
  • C08L2666/00—Composition of polymers characterized by a further compound in the blend, being organic macromolecular compounds, natural resins, waxes or and bituminous materials, non-macromolecular organic substances, inorganic substances or characterized by their function in the composition
  • C08L2666/54—Inorganic substances

Definitions

• This invention pertains to the use of epoxy resins in adhesive compositions with improved properties.
• the invention also provides for a physically stable dispersion which may be used in such adhesive compositions.
• Epoxy resins have a spectrum of properties that are well adapted for use in coatings, adhesives, fiber-reinforced laminates, composites, engineering plastics and specialty applications, such as potting resins and mortars. Such properties include excellent strength, toughness, corrosion and solvent resistance, good adhesion and electrical properties, good dimensional stability, hardness, and low shrinkage on cure.
• Epoxy-based adhesives offer many advantages to end-users such as high strength a nd moisture resistance while providing protective seals as well as load-bearing properties.
• epoxy resins are formulated into either one- or two-component, ambient or heat-cured paste or film adhesives.
• Typical ingredients, in addition to the epoxy resin, include curing agents, catalysts and accelerators, fillers, pigments, reactive diluents, .nonreactive diluents, solvents, flexibilizers, toughening agents, extenders and rheological control agents.
• epoxy resins toughened with insoluble dispersions of acrylic elastomers
• adhesive formulations based upon epoxy resins, toughened with insoluble dispersions of acrylic elastomers show poor rheological control.
• sag 0 control can be partially or completely achieved by room temperature cure.
• rheological control is achieved by rapid build-up of microstructure. If rheological control is primarily achieved by microstructure build-up at ambient conditions, a balance between cure rate and dispensing rate must be maintained.
• the present invention solves some of those problems described above by disclosing a thixotropic adhesive composition which has improved rheological control and sag resistance over known adhesives.
• This invention is a thixotropic adhesive composition exhibiting good sag resistance comprising a
• the invention comprises a mixture of a dispersion which comprises an uncured epoxy resin as a continuous phase having dispersed therein an in situ polymerized
• the invention is also a method of preparing an 0 adhesive composition with superior rheological control and toughness at lower viscosities compared to existing one- or two-part toughened epoxy resin adhesives.
• the invention shows a method of adhering surfaces together using an epoxy resin based adhesive. 5
• the method of preparing the adhesive composition comprises (a) preparing a dispersion which comprises an uncured epoxy resin as a continuous phase having dispersed therein an in situ polymerized 0 insoluble acrylic elastomer which has a plurality of hydroxyl functionality, and (b) mixing together the dispersion and an effective amount of a rheological control agent which has a plurality of pendent hydroxyl groups.
• a rheological control agent should be capable of rheological control without sag or stringing at relatively low, easy to pump viscosity levels.
• an epoxy resin curing agent may be added to the composition during the mixing together of the dispersion and the rheological control agent.
• the method of using the adhesive composition comprises first applying the adhesive composition to a first surface; contacting the first surface having the adhesive composition applied thereto to a second surface in a bonding relationship with the adhesive composition disposed between the surfaces; and curing the adhesive composition.
• An epoxy resin cure accelerator can also be optionally used for rapid cure.
• the invention requires a novel polymer-modified epoxy resin in which acrylic elastomer particles have added hydroxyl functionality.
• the main function of the acrylic elastomer is to improve toughness. Toughness of the epoxy resin is accomplished by selecting an optimum dispersed polymer phase and by balancing that with rheological control agents, thixotropes, fillers, curing agents, and other additives. Rheological control is achieved specifically by matching the hydroxyl groups of the rheological control agent with the hydroxyl functionality of the dispersed polymer phase of the epoxy resin. Hydroxyl functionality groups can be applied in different concentration levels, can be selected from a number of different chemical structures, preferably of acrylic or polyurethane composition, and can be grafted onto different moieties in the dispersed polymer phase.
• the adhesive of the present invention has improved rheological control and adhesive performance while maintaining polymer properties such as low viscosities, high glass transition temperatures, toughness, and moisture resistance.
• the invention reduces the amount of thixotrope needed, extending the shelf life of common thixotropes giving reliable rheological properties, and reduces raw material costs.
• adhesive systems engineered according to the invention show better rheological and sag control characteristics compared to adhesives prepared heretofore with liquid reactive resins, such as carboxyl-terminated butadiene- -acrylonitrile liquid polymers, liquid epoxy resins and unmodified insoluble polymer dispersions in liquid epoxy resins.
• liquid reactive resins such as carboxyl-terminated butadiene- -acrylonitrile liquid polymers, liquid epoxy resins and unmodified insoluble polymer dispersions in liquid epoxy resins.
• Adhesives engineered according to the invention exhibit excellent sag control characteristics over at least several months at temperatures ranging from ambient temperature up to 450°F (232°C).
• the invention eliminates the use of additional diluents to incorporate large quantities of fumed silica for the purpose of sag control. Based on resin compositions without diluents, lower viscosities for ease of handling are possible. Glass transition temperatures are higher and moisture resistance is improved over known epoxy resin adhesives.
• Adhesives prepared according to the invention are also more economical since cheaper grades of hydrophilic fumed silica can be used instead of the more expensive grades.
• the present invention has also solved rheological control problems in paste-type toughened epoxy adhesives.
• the invention has immediate use in adhesive and sealant applications, especially in structural adhesives.
• Other epoxy resin applications where rheological control is important include coatings, civil engineering applications, laminates, composites, reinforced plastics, and electrical encapsulations.
• the present invention is also a stable dispersion of an organic polymer in an epoxy resin as a continuous phase, characterized in that the dispersion remains insoluble in the epoxy resin at a temperature of at least 60°C.
• the dispersed phase can be prepared by the polymerization of one or more monomers in a step reaction, in an addition reaction such as a cationic, anionic, or coordination polymerization, or free radical chain addition.
• the dispersed phase is the polymerizate of an in situ polymerized ethylenically unsaturated functional monomer. It is also preferred that the dispersion contains a dispersion stabilizer.
• the dispersion comprises an uncured epoxy resin as a continuous phase having dispersed therein an in situ polymerized insoluble acrylic elastomer which has a hydroxyl functionality and a dispersion stabilizer which has a moiety compatible with the epoxy resin and a moiety compatible with the in situ polymerized insoluble acrylic elastomer.
• epoxy resin is employed to designate a compound or mixture containing, on an average, greater than one 1,2-epoxy group per molecule, which can be crosslinked into final form by means of a chemical reaction with a variety of curing agents used with or without heat.
• 1,2-epoxy group is a glycidyl residue:
• ⁇ / 0 such as is found in glycidyl ethers, glycidyl esters or glycidyl amines.
• epoxy resins include diglycidyl ether of bisphenol A and tetraglycidyl- methylenedianiline.
• lower viscosity epoxy resins are used for paste adhesives.
• the 5 materials can be diluted with reactive diluents, typically mono-functional, low viscosity epoxides.
• the epoxy resin is a liquid epoxy resin at ambient temperature.
• a polymer-modified epoxy resin is used.
• a dispersion which comprises an uncured epoxy resin as a continuous phase having dispersed therein as a discontinuous phase an -*,- in situ polymerized insoluble acrylic elastomer which has a plurality of hydroxyl functionality is used.
• the term "in situ polymerized insoluble acrylic elastomer" 0 is employed to designate a discontinuous particulate phase which is made by polymerizing or copolymerizing vinyl monomers in a continuous epoxy resin phase.
• the particulate phase can be prepared by the polymerization of one or more monomers in a step reaction 5 (condensation), in an addition reaction such as a cationic, anionic, or coordination polymerization, or free radical chain addition.
• the particulate phase is normally elastomeric 0 if the glass transition temperature (T g ) is below room temperature.
• T g glass transition temperature
• low glass transition temperatures, below room temperature are obtained by including alkyl esters of acrylic or methacrylic acid as monomers.
• acrylic acid is used.
• the alkyl group of such alkyl esters of acrylic or methacrylic acids contains at least 4 carbon atoms and more preferably 4 to 8 carbon atoms.
• butyl acrylate and 2-ethylhexyl acrylate are preferred.
• the dispersed phase can be in an amount of from 5 to 70 weight percent, preferably 5 to 50 weight percent, most preferably 5 to 20 weight percent, of the total dispersion as long as the epoxy resin is the continuous phase.
• the optimum concentration of the polymeric dispersed phase can and will be varied depending upon the materials employed and the end-use that is envisaged.
• the dispersions are usually made at a solids level at which the dispersions are to be used. However, it is possible to prepare dispersions of higher solids level and dilute to the final solids level.
• the dispersions are more easily prepared and have superior stability and other properties when a dispersion stabilizer is included in the composition.
• the dispersion stabilizer can be any compound which contains at least two different moieties in its molecule, with at least one moiety compatible with the epoxy resin and at least one other moiety compatible with the in situ polymerized insoluble acrylic elastomer.
• compatible in reference to the moieties is meant to designate that a moiety is miscible or soluble in a phase of the dispersion.
• a preferred dispersion stabilizer is the polymerizate of at least one vinyl monomer and a vinylized epoxy resin adduct.
• a process for preparing the dispersion described above is characterized by the steps of (1) providing a vinylized epoxy resin adduct by reacting a minor amount of functional monomer with a polyepoxide continuous phase, (2) providing a dispersion stabilizer by reacting the adduct with at least one vinyl monomer and (3) polymerizing said vinyl monomers in the polyepoxide continuous phase and in the presence of said dispersion stabilizer.
• steps (2) and (3) are performed at the same time.
• the dispersion stabilizer is prepared separately and added to the polyepoxide before or during the addition and polymerization of the vinyl monomer.
• the vinylized epoxy resin adduct is the reaction product of a functional monomer with an epoxy resin.
• a functional monomer has a reactive group in addition to a polymerizable double bond.
• the vinylized adduct is made by reaction of a functional monomer which is reactive with an oxirane group of an epoxy resin.
• the reactive group may be, for example, the active hydrogen in a carboxylic acid, phenol, thiophenol, isocyanate, or an amine group.
• the functional monomer is an ethylenically unsaturated functional monomer.
• a preferred functional monomer is a substituted carboxylic acid.
• the reaction product is an acrylic or methacrylic acid ester of the diglycidyl ether of bisphenol A.
• the term "adhesive” is employed to designate a formulation which is capable of bonding other substances together by surface attachment.
• the term "rheological control agent” is employed to designate a material which affects the deformation and flow of an adhesive in terms of stress, strain and time. Any acceptable level of rheological control agent may be used in the dispersion, but typically the rheological control agent is used in a level of up to 10 percent by weight of the total adhesive composition.
• An effective amount of a rheological control agent will be an amount necessary to achieve a desired level of deformation and flow of an adhesive and will generally be predetermined by the formulators of the adhesive. Such deformation and flow properties of an adhesive will generally be dictated by specific consumer or industrial need requirements and, as such, will vary from product to product.
• Typical rheological control agents have included fumed silica, asbestos, carbon black, clays and chopped Kevlar fibers. Because of the carcinogenicity of asbestos, ineffectiveness of clays and the expense of Kevlar fibers, the most typical rheological control agents are fumed silicas such as those made by Cabot Corporation and sold under the trade name of CAB-0-SILTM. In the present invention, hydrophilic fumed silica is preferred.
• hydrophilic fumed silica results from its unusual surface which is populated by hydrogen-bonded hydroxyl groups, pendant hydroxyl groups, and siloxane groups.
• the pendant hydroxyl groups contribute greatly to the unique behavior of fumed silica because they can bond to pendant hydroxyl groups on other hydrophilic fumed silica particles forming a temporary three-dimensional network of particles that will pervade a liquid system.
• the hydrogen bonding is easily broken under shear stress during mixing and the viscosity decreases. But these bonds reform and the viscosity increases again (“thixotropy”) . See, for example, the Cabot Corporation's "CAB-0-SILTM Fumed Silica Properties and Functions" (1990) booklet.
• hydrophobia fumed silica is fumed silica which has been reacted with a compound to form a treated fumed silica.
• Typical compounds used to form treated fumed silicas include dimethyldichlorosilane and hexamethyldisilazane. Treatment with such compounds replaces many of the surface hydroxyl groups on the fumed silica with other groups, such as methyl groups. Any remaining surface hydroxyl groups typically become effectively shielded from interactions with other hydroxyl groups due to the added groups. See, for example, the Cabot Corporation's booklet "CAB-0-SILTM TS-720 Treated Fumed Silica" (1990).
• hydroxyl functionality is employed to designate the presence of the hydroxyl chemical group (-0H).
• This functionality can be introduced in the in situ polymerized insoluble acrylic elastomer by adding a hydroxyl functional vinyl comonomer such as hydroxyethyl acrylate, hydroxypropyl acrylate and methacrylate and hydroxybutyl acrylate and methacrylate.
• a hydroxyl functional vinyl comonomer such as hydroxyethyl acrylate, hydroxypropyl acrylate and methacrylate and hydroxybutyl acrylate and methacrylate.
• Almost any level of hydroxyl functional vinyl comonomer may be used so as to achieve a desired level of rheology control.
• low levels of hydroxyl functional vinyl comonomer should be used so as to achieve good rheology control but so as not to adversely affect other properties of the epoxy resin.
• the hydroxyl functional vinyl comonomer can be in an amount of from 1 to 10
• thixotropes is employed to designate materials which impart the ability of certain colloidal gels to liquify under stress. Examples include clays.
• fillers is employed to designate inert materials which are used to provide a certain degree of stiffness and hardness and tb decrease the cost of the product. Examples include calcium carbonate, silicates, and soft clays. Any acceptable level of filler may be used in the adhesive composition, but typically the filler is used in a level of up to 50 percent by weight of the total adhesive composition.
• curing is employed to designate the conversion of a raw resin product to a finished and useful condition, usually by application of heat and/or chemicals which induce physico-chemical changes.
• curing agent is employed to designate active hydrogen-containing compounds such as amines, amides, mercaptans, acids, phenolics, alcohols, anhydrides, Lewis acids, and bases which are added to a formulation to aid in curing of the formulation.
• Dicyandiamide is a preferred curing agent.
• any acceptable level of curing agent may be used in the adhesive composition, but typically the curing agent is used in a catalytic level up to a stoichiometric level based on the active hydrogen content of the adhesive composition, although a slight excess of curing agent may be used.
• additives is employed to designate materials such as surfactants, antioxidants, stabilizers, colorants, inhibitors, and plasticizers which are added to a formulation as dictated by need requirements. Any acceptable level of other additives may be used in the adhesive composition, but typically the other additives are used in a level of up to
• Sagging is defined as run or flow-off of adhesive from an adherent surface due to application of excess or low-viscosity material.
• a typical example of a sag test can be found in General Motors Engineering Standards Bulletin, "Sag Test For Structural Adhesives, GM9749P", available from General Motors Corporation.
• HEMA hydroxyethyl methacrylate
• the resin is heated with stirring under an air atmosphere to 120°C. After an additional 60 minutes, analysis of the mixture by titration with standard base demonstrates that greater than 97 percent of the methacrylic acid has reacted with the epoxy resin to form a vinyl ester.
• a mixture (monomer/initiator solution) of 2-ethylhexyl acrylate (291 g), glycidyl methacrylate (9 g), tert-butyl peroctoate (3 g), and tert-butyl perbenzoate (1.5 g) is added to the modified epoxy resin at 120°C over approximately a 60 minute period.
• the temperature is held at 120°C for an additional three hours. Additional tert-butyl perbenzoate (0.6 g) is then added.
• the reactor temperature is raised to 140°C and held for an additional two hours.
• the product is cooled and bottled.
• the final product is a stable dispersion of acrylic elastomer in a liquid epoxy resin.
• the product has a hydroxyl functionality due to the hydroxyl groups bound to the insoluble acrylic rubber particles in the curable epoxy base resin.
• the product has a Brookfield viscosity of 84,000 cps (84,000 mPa.s) at 25°C and an epoxide equivalent weight of 240.
• dispersion of vinyl polymer in an epoxy resin is prepared using techniques and materials similar to those described hereinbefore in Example 1 , except that no hydroxyethyl methacrylate is added to the epoxy resin.
• an unmodified sample of a diglycidyl ether of bisphenol A having an epoxide equivalent weight of from 176 to 186 and a viscosity at 25°C of between 9,000 and 11,500 cps (9,000 and 0 11,500 mPa.s) is used.
• a diglycidyl ether of c bisphenol A having an epoxide equivalent weight of from 172 to 176 and a viscosity at 25°C of between 4,000 and 6,000 cps (4,000 and 6,000 mPa.s) and sold commercially by The Dow Chemical Company as TACTIXTM 123 epoxy resin is used.
• Example 1 modified without hydroxyethyl methacrylate, has a higher viscosity after being blended with hydrophilic fumed silica but still has poor rheological control.
• Example 1 modified with hydroxyethyl methacrylate, has both a high viscosity and good rheological control.
• Example 1 The composition of Example 1 (375 g, HEMA-modified poly(2-ethylhexyl acrylate) elastomer in D.E.R.TM 383), calcium carbonate (100 g), and hydrophilic fumed silica (25 g) are mixed together in a mixer. Dicyandiamide, CG-1200 grade (available from Air
• D.E.R.TM 383 liquid epoxy resin (375 g), calcium carbonate (100 g), and hydrophilic fumed silica (25 g) are blended together.
• Dicyandiamide CG-1200 grade is used as a curing agent at 95 percent of stoichiometric ratio and a p,p'-methylene bis(phenyldimethylurea) catalyst at 6 phr level is used to accelerate cure characteristics.
• a dynamic strain sweep measurement experiment is conducted at a constant rate of 1 radian per second on the adhesive compositions of Example 2 and Comparative Example D after respectively different storage times. Yield values are measured at ambient temperature using the Fluids Rheometer RF 7800 by Reometrics, Inc., Piscataway, New Jersey.
• the adhesive of Comparative Example D behaves more like a liquid than a solid (i.e., G' is significantly less than G").
• the adhesive of Comparative Example D does not have a yield value and fails the sag test. before the test is started both at room temperature and at 155°C.
• Comparative Example A 375 g, poly(2-ethylhexyl acrylate) elastomer in D.E.R.TM 383), calcium carbonate (100 g), and hydrophilic fumed silica (25 g) are mixed together.
• Dicyandiamide CG-1200 grade
• a p,p'-methylene bis(phenyldimethylurea) catalyst at 6 phr level is used to accelerate cure characteristics.
• a one part adhesive available from PPG Industries, Inc. as HC6227 Structural Epoxy Adhesive, is used as a further comparative.
• CYBONDTM 4551G Adhesive is used as a further comparative.
• CYBOND is a trademark of the American Cyanamid Company.
• Example 2 The adhesives of Example 2 and Comparative Examples D, E, F and G are subjected to tests to determine the lap shear strength, side impact strength, 5 side impact strength failure mode, T-peel strength, and T-peel strength failure mode.
• Example 2 adhesive Yield values were measured at ambient temperature, at a rate of 1 radian/second using the Fluids Rheometer RF 7800 by Reometrics, Inc., Piscataway, New Jersey. The exact yield value of the Example 2 adhesive cannot be measured because it exceeds the upper limit of the measuring capabilities of the instrument at 1 radian/second. Comparative Example G adhesive yield value measurement made at 0.5 radian/second gives 1.04 x 1 ⁇ 3 dynes (1.04x10 ⁇ 2 N)/square centimeter, and at 5.0 radian/second gave 2.40 x 1 ⁇ dynes (2.40x10 ⁇ 2 N)/square centimeter.
• Test Conditions 63 mil (1.6x10-3 m) cold-rolled steel (CRS) substrate, ground-to-ground, 0.5 weight percent 4 mil (1x10 -2 * m) glass beads, 0.1 in (2.5x10-3 m)/min crosshead speed; ASTM Test Method D-1002 TABLE V
• Test Conditions Charpy Side Impact Strength on 63 mil (1.6x10-3 m) CRS substrate, ground-to-ground, 0.5 weight percent 4 mil (IxlO- 1 * m) glass beads.
• Test Conditions Charpy Side Impact Strength on 63 mil (1.6x10-3 m) CRS substrate, ground-to-ground, 0.5 weight percent 4 mil (IxlO- 2 * m) glass beads.
• Test Conditions 32 mil (8x10 " ⁇ m) one-fourth hardness CRS. ground-to-ground, 0.5 weight percent 4 mil (1x10" ⁇ m) glass beads, 10.0 in (0.25 m)/min crosshead speed; ASTM Test Method D-1876
• Test Conditions 32 mil ( ⁇ xlO- 1 * m) one-fourth hardness CRS . ground-to-ground, 0.5 weight percent 4 mil ( 1x10-4 m ) glass beads , 10.0 in (0.25 m) /min crosshead speed

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• Chemical & Material Sciences (AREA)
• Organic Chemistry (AREA)
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• Chemical Kinetics & Catalysis (AREA)
• Medicinal Chemistry (AREA)
• Polymers & Plastics (AREA)
• Adhesives Or Adhesive Processes (AREA)
• Epoxy Resins (AREA)
• Compositions Of Macromolecular Compounds (AREA)

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