STRUCTURAL ADHESIVES CONTAINING MALEIMIDE TERMINATED
POLYIMIDES
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims priority to U.S. Provisional Application No. 61/149,108,
for "STRUCTURAL ADHESIVES CONTAINING IMIDE-EXTENDED MALEIMIDE",
filed February 2, 2009, and which is herein incorporated by reference in its entirety.
FIELD OF THE INVENTION
The present invention relates to structural adhesive compositions. More
particularly, the invention relates to structural adhesives having improved strength and
toughness.
BACKGROUND OF THE INVENTION
Acrylic structural adhesive compositions are well-known articles of commerce
which are extensively used commercially for bonding metal and plastic materials. The
load-bearing and stress-relieving properties of structural adhesives, as well as their bond
strength, which can exceed the strength of the materials which are being bonded, make
these adhesives attractive alternatives to or partial replacements for mechanical methods,
such as riveting or spot welding, of joining materials, especially where it is preferable to
distribute load stresses over larger areas rather than to concentrate such stresses at a few
points. Their use can reduce or eliminate costly finishing operations necessitated by
mechanical joining methods, present a more pleasing exterior and at least reduce the
possibility of corrosion of assemblies containing one or more metal components.
Additionally, they can be used to bond a diversity of metals without extensive surface
preparation.
Acrylic structural adhesives are extensively used for providing structural strength-
imparting bonds to joined metal and or polymer materials. Acrylic structural adhesives are
useful for bonding of metal parts in place of welding or mechanical fastening techniques.
The structural requirements include high bond strength and good failure mode. A typical
method to measure bond strength is the lap shear, high speed impact, and T-peel tests. One
prevalent use for acrylic structural adhesives is in forming hem flanges in automotive body
panels and doors. Exemplary conventional acrylic structural adhesives and methods for
using acrylic structural adhesives are disclosed in the following U.S. Patents: U.S. Pat. No.
6,180,199 entitled Beaded Adhesive And Hem Flanged Part Made Therefrom; U.S. Pat.
No. 6,074,506 entitled Method Of Bonding Using Non-Compressible Beads; U.S. Pat. No.
5,932,638 entitled Free, Radical Polymerizable Compositions Including Para-Halogenated
Aniline Derivatives; U.S. Pat. No. 5,783,298 entitled Adhesive Mixture With Non-
Compressible Beads Therein U.S. Pat. No. 5,710,235 entitled Olefinic And Urethane-
Terminated Ester Polyalkadiene; U.S. Pat. No. 5,641,834 entitled Modified Polyalkadiene-
Containing Compositions; and U.S. Pat. No. 5,632,413 entitled Adhesive Bonding
Apparatus And Method Using Non-Compressible Beads. Conventional acrylic structural
adhesives typically comprise a mixture of one or more olefinic reactive monomers such as
methyl methacrylate and methacrylic acid, toughener(s) and redox initiator system. The
toughener(s), which may or may not be reactive, or polymerizable with the reactive
monomers. Reactive polymers such as unsaturated polyesters and acrylourethane
prepolymers may be used to graft onto or crosslink the initiated monomers during
polymerization. In addition, fully formulated acrylic structural adhesives typically contain
other additives for improving adhesion to substrate materials, environmental resistance,
impact strength, flexibility, heat resistance, and the like. Epoxy resins impart improved
heat resistance.
Unfortunately, with many prior art structural adhesives there is a trade-off between
flexibility/toughness and strength. In order to increase flexibility and toughness rubbery
polymers are added to the adhesive, however the addition of these rubbery polymers
negatively impacts the modulus and ultimately the strength of the adhesive. It would
therefore be desirable to provide a structural adhesive formulation with increased
flexibility and toughness without sacrificing strength.
It is to these needs that the present invention is directed.
SUMMARY OF THE INVENTION
In a first aspect of the present invention, a structural adhesive is provided
comprising a free radical-polymerizeable monomer, and a maleimide terminated
polyimide wherein the maleimide comprises a mono-, bis-, or poly-maleimide compound.
In one embodiment of the present invention, the radical-polymerizable monomer
comprises THFMA. In another embodiment of the present invention, the adhesive further
comprises a toughener, preferably comprising at least one of GMA/CTB adducts, core-
shell impact modifiers, and block copolymer elastomers.
In an additional embodiment of the present invention, the adhesive further
comprises an adhesion promoter, preferably HEMA phosphate. In yet another
embodiment of the present invention, the adhesive further comprises a metal
dimethacrylate, preferably zinc dimethacrylate. In a further embodiment of the present
invention, the adhesive further comprises ethoxylated bisphenol A dimethacrylate. And in
another embodiment of the present invention, the adhesive further comprises a particulate
additive comprising at least one of calcium metasilicate or fumed silica.
In an additional aspect of the present invention, a two part structural adhesive
composition is provided comprising in part A: (a) at least one free radical-polymerizable
monomer; (b) a maleimide terminated polyimide; and (c) a reducing agent, and in part B:
an oxidizing agent. In a preferred embodiment of the present invention, the reducing
agent comprises at least one of N,N-diisopropanol-p-chloroaniline; N,N-diisopropanol-p-
bromoaniline; N,N-diisopropanol-p-bromo-m-methylaniline; N,N-dimethyl-p-
chloroaniline; N,N-dimethyl-p-bromoaniline; N,N-diethyl-p-chloroaniline; N,N-diethyl-p-
bromoaniline; N,N-dimethyl-p-aniline; and N,N-diisopropanol-p-toluidine. And in
another embodiment of the present invention, the oxidizing agent comprises
benzoylperoxide. In a preferred embodiment of the present invention, the weight ratio of
the first package to the second package is from about 1:1 to about 15:1.
In a further aspect of the present invention, a method for bonding two substrates is
provided comprising the steps of providing a structural adhesive composition comprising a
maleimide terminated polyimide on a first substrate, and contacting the second substrate to
the structural adhesive on the first substrate and curing the structural adhesive. In another
embodiment of the present invention, the structural adhesive comprises at least one of an
acrylic-based structural adhesive or an epoxy-based structural adhesive.
DETAILED DESCRIPTION OF THE INVENTION
In a first aspect of the present invention, a structural adhesive is provided
comprising a maleimide terminated polyimide, preferably a bismaleimide terminated
polyimide. The structure of this additive provides improved strength, toughness, and
thermal stability to final polymer network.
In a further aspect of this invention, a maleimide terminated polyimide is
incorporated into a two-part acrylic adhesive system for improving thermal stability,
strength, and toughness. It is believed that the maleimide terminated polyimide of the
various embodiments of the present invention provide this balance of physical features
through the combination of rigid aromatic sections interspersed with flexible aliphatic
chains.
The maleimide terminated polyimide is a unique molecule that possesses both rigid
imide segments and flexible dimer-fatty acid chains. When formulated into an acrylic
adhesive, it enhances both strength and toughness when maintaining the thermal stability.
The A-side of the two part reactive acrylic structural adhesive contains 10 to 90%
by weight of at least one free radical-polymerizable monomer in a major amount (the
primary monomer). Representative monomers include esters of (meth)acrylic acid such as
methyl methacrylate, ethyl methacrylate, butyl methacrylate, methyl acrylate, butyl
acrylate, cyclohexyl acrylate, hexyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, ethyl
acrylate, diethylene glycol dimethacrylate, dicyclopentadienyloxyethyl methacrylate,
cyclohexyl methacrylate, lauryl methacrylate, glycidyl methacrylate and tetrahydrofurfuryl
methacrylate (THFMA). The preferred monomer(s) contributes rigidity in the cured
polymer and is selected from methacrylic esters exhibiting a homopolymer Tg of at least
50 °C, preferably 60 °C, and some as much as 105 °C.
In a further embodiment of the present invention, the primary monomer may be
combined with an ethylenic unsaturated carboxylic monomer such as methacrylic acid,
acrylic acid, substituted (meth)acrylic acids such as itaconic acid, maleic acid and fumaric
acid. Further optional comonomers includable herein are acrylonitrile, methacrylonitrile,
acrylamide and methacrylamide; vinyl acetate; vinylidene chloride; and butadienes such as
2,3-dichloro-l,3-butadiene and 2-chloro-1,3-butadiene. Other useful monomers include
maleate esters; and fumarate esters. In one embodiment, a mixture of the monomers
tetrahydrofurfuryl methacrylate, methacrylic acid and methyl methacrylate is useful. In
further embodiments it is optionally preferable to include a reactive diluent with the
primary monomer.
Comonomers optionally includable with the primary monomer include OH-
functional monoethylenic unsaturated monomers like 2-hydroxyethyl(meth)acrylate, 3-
hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 4-
hydroxycyclohexyl(meth) acrylate, 1,6-hexanediol mono(meth) acrylate, neopentyl glycol
mono(meth)acrylate, 1,6-hexanediol dimethacrylate, and 1,4-butanediol dimethacrylate.
Preferedly from 0.0 to 10 wt % (on wt. of A-side) of a multifunctional crosslinking
comonomer is included, such as trimethylohpropane di(meth) acrylate, trimethylolethane
di(meth) acrylate, pentaerythritol tri(meth) acrylate, dipentaerythritol penta(meth)acrylate,
and epoxy-diacrylates, such as ethoxylated Bisphenol A dimethacrylate.
The maleimide terminated polyimide employed in the present invention comprises
any maleimide terminated polyimide. In one embodiment of the present invention, the
maleimide portion(s) comprise mono-, bis-, or poly-maleimide. In a preferred
embodiment of the present invention, the maleimide terminated polyimide comprises those
disclosed in U.S. Patents No. 7,157,587 and 7,208,566 herein incorporated by reference in
full.
In one embodiment of the present invention, the molecular weight (average
molecular weight) of the maleimide terminated polyimide is at least 2,000. In a preferred
embodiment of the present invention, the molecular weight is at least about 3,000. In a
most preferred embodiment of the present invention, the molecular weight is less than
about 10,000, and most preferably less than about 5,000.
In a further embodiment of the present invention, the maleimide terminated
polyimide is present in the A side of the adhesive in an amount from about 5 to about 30
weight percent, and more preferably from about 10 to about 20 weight percent.
In a further embodiment of the present invention, the adhesive further comprises an
epoxy compound. The epoxy compound of embodiments of the present invention
comprises any material that contains an epoxy (oxirane) group. Included epoxy resins are
epoxy cresol novolacs, epoxy phenol novolacs and blends of either of these with bisphenol
A epoxy resins. Monomelic epoxy compounds and epoxides of the polymeric type can be
aliphatic, cycloaliphatic, aromatic or heterocyclic. The "average" number of epoxy groups
per molecule is determined by dividing the total number of epoxy groups in the epoxy-
containing material by the total number of epoxy molecules present. Useful epoxy
materials generally contain on the average at least 1.5 polymerizable epoxy groups per
molecule. Preferably two or more epoxy groups per molecule are present. The polymeric
epoxides include linear polymers having terminal epoxy groups (e.g., a diglycidyl ether of
a polyoxyalkylene glycol), polymers having skeletal oxirane units (e.g., polybutadiene
polyepoxide), and polymers having pendent epoxy groups (e.g., a glycidyl methacrylate
polymer or copolymer). The epoxides may be pure compounds but are generally mixtures
containing one, two, or more epoxy groups per molecule.
The epoxy-containing materials may vary from low molecular weight monomeric
materials to high molecular weight polymers, and may vary greatly in the nature of their
backbone and substituent groups. For example, the backbone may be of any type, and
substituent groups thereon may be free of an active hydrogen atom. Illustrative of
permissible substituents groups include halogens, ester groups, ethers, sulfonate groups,
siloxane groups, nitro groups, phosphate groups, etc. The molecular weight of the epoxy-
containing materials may vary from about 50 to 100,000 or more. Mixtures of various
epoxy-containing materials can also be used in the compositions of this invention.
The epoxy compounds of the present invention can be cycloaliphatic epoxides.
Examples of cycloaliphatic epoxides include diepoxides of cycloaliphatic esters of
dicarboxylic acids such as bis(3,4-epoxycyclohexylmethyl)oxalate, bis(3,4-
epoxycyclohexylmethyl)adipate, bis(3,4-epoxy-6-methylcyclohexylmethyi)adipate,
bis(3,4-epoxycyclohexylmethyl)pimelate, and the like. Other suitable diepoxides of
cycloaliphatic esters of dicarboxylic acids are described in, for example, U.S. Pat. No.
2,750,395, which is incorporated herein by reference.
Other cycloaliphatic epoxides include 3,4-epoxycyclohexylmethyl-3,4-
epoxycyclohexane carboxylates such as 3,4-epoxycyclohexylmethyl-3,4-
epoxycyclohexane carboxylate; 3,4-epoxy-1 -methylcyclohexy lmethy 1-3,4-epoxy-1 -
methylcyclohexane carboxylate; 6-methyl-3,4-epoxycyclohexylmethyl-6-methyl-3,4-
epoxycyclohexane carboxylate; 3,4-epoxy-2-methylcyclohexylmethyl-3,4-epoxy-2-
methylcyclohexane carboxylate; 3,4-epoxy-3-methylcyclohexylmethyl-3,4-epoxy-3-
methylcyclohexane carboxylate; 3,4-epoxy-5-methylcyclohexylmethyl-3,4-epoxy-5-
methylcyclohexane carboxylate and the like. Other suitable 3,4-epoxycyclohexylmethyl-
3,4-epoxycyclohexane carboxylates are described in, for example, U.S. Pat. No.
2,890,194, which is incorporated herein by reference.
Epoxy resins based on bisphenol A, either solids, per se, and capable of dissolution
in a carrier, or liquids per se, are preferred as these are relatively inexpensive. There are a
myriad of available epoxy materials, collectively referred to as epoxy resins whether
resinous or simple compounds. In particular, simple epoxy compounds which are readily
available include octadecylene oxide, glycidylmethacrylate, diglycidyl ether of bisphenol
A (e.g., those available under the trade designations EPON from Shell Chemical Co.,
DER, from Dow Chemical Co.), vinylcyclohexene dioxide (e.g., ERL-4206 from Union
Carbide Corp.), 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate (e.g.,
ERL-4221 from Union Carbide Corp.), 3,4-epoxy-6-methylcyclohexylmethyl-3,4-epoxy-
6-methylcyclohexene carboxylate (e.g., ERL-4201 from Union Carbide Corp.), bis(3,4-
epoxy-6-methylcyclohexylmethyl) adipate (e.g. ERL-4289 from Union Carbide Corp.),
bis(2,3-epoxycyclopentyl) ether (e.g., ERL-0400 from Union Carbide Corp.), aliphatic
epoxy modified with polypropylene glycol (e.g., ERL-4050 and ERL-4052 from Union
Carbide Corp.), dipentene dioxide (e.g., ERL-4269 from Union Carbide Corp.),
epoxidized polybutadiene (e.g., OXIRON 2001 from FMC Corp.), silicone resin
containing epoxy functionality, flame retardant epoxy resins (e.g., DER-580, a brominated
bisphenol type epoxy resin available from Dow Chemical Co.), 1,4-butanediol diglycidyl
ether of phenolformaldehyde novolak (e.g., DEN-431 and DEN-438 from Dow Chemical
Co.), and resorcinol diglycidyl ether.
Still other epoxy-containing materials are copolymers of epoxy (meth)acrylic acid
esters, such as glycidylacrylate and glycidylmethacrylate with one or more
copolymerizable vinyl compounds. Examples of such copolymers are 1:1 styrene-
glycidylmethacrylate, 1:1 methylmethacrylateglycidylacrylate and a 62.5:24:13.5
methylmethacrylate-ethylacrylateglycidylmethacrylate.
In one embodiment of the present invention, the epoxy is present in the final
formulation in an amount from about 2 to about 20 weight percent and preferably from
about 3 to about 10 weight percent. In an alternate embodiment of the present invention,
the structural adhesive comprises an epoxy-based structural adhesive wherein an epoxy
resin is the primary adhesive material such as those described in U.S. Patents No.
4,578,424 and 5,385,990, herein incorporated by reference.
Any known suitable toughener can be utilized in the structural adhesives according
to the inventions. The toughener examples include various solid and liquid elastomeric
polymeric materials, and in particular liquid olefinic-terminated elastomers as described in
U.S. Pat. Nos. 4,223,115; 4,452,944; 4,769,419; 5,641,834 and 5,710,235; and olefmic
urethane reaction products of an isocyanate-functional prepolymer and a hydroxy
functional monomer, as described in U.S. Pat. Nos. 4,223,115; 4,452,944; 4,467,071 and
4,769,419, the entire disclosure of each which is hereby incorporated by reference.
Preferred urethane modified olefinic-terminated liquid elastomers include those
disclosed in U.S. Patent No. 4,769,419 comprising the reaction product of an olefinic
monoepoxide compound with a polycarboxylic homopolymer of conjugated dienes, and
most specifically, the glycidyl methacrylate / carboxyl terminated butadiene (GMA/CTB)
adduct as described in US Patent No. 4,769,419, Example 1.
A-B-A triblock block copolymers are useful tougheners. In one example the A
block is polystyrene, alpha-methyl styrene, t-butyl styrene, or other ring alkylated styrenes
as well as mixtures of some or all of the above and the B block is an elastomeric segment
having a Tg of 0° C. or less, such as that derived from a conjugated diene, like butadiene,
isobutylene or other olefin, like ethylene-propylene monomer. Commercially available
block copolymer tougheners include EUROPRENE® which are available from Enichem
Elastomers Americas, Inc. A preferred toughener is based on a terblock polymer of
styrene-[isoprene]-styrene, 25-[50]-25, parts by weight. Other high molecular weight
tougheners include, for example, block copolymers and random copolymers including but
not limited to polyethylene, polypropylene, styrene-butadiene, polychloroprene, EPDM,
chlorinated rubber, butyl rubber, styrene/butadiene/acrylonitrile rubber and
chlorosulfonated polyethylene.
Other tougheners include the liquid olefinic-terminated elastomers, wherein the
elastomeric moiety is based on homopolymers of butadiene, copolymers of butadiene and
at least one monomer copolymerizable therewith, for example, styrene, acrylonitrile,
methacrylonitrile (e.g. poly(butadiene-(meth)acrylonitrile or poly(butadiene-
(meth)acrylonitrile-styrene) and mixtures thereof; as well as modified elastomeric
polymeric materials, such as butadiene homopolymers and copolymers modified by
copolymerization therewith of trace amounts of up to about 5 percent by weight of the
elastomeric material of at least one functional monomer (such as acrylic acid, methacrylic
acid, maleic anhydride, fumaric acid, styrene, and methyl methacrylate to give, for
example, methacrylate-terminated polybutadiene homopolyrniers and/or copolymers.
Inclusive as tougheners are the olefinic-terminated polyalkadienes having carboxy
ester linking groups and at least one nascent secondary hydroxyl group, such as disclosed
in U.S. Pat. No. 5,587,433, incorporated therein by reference. The secondary OH group
may be optionally caped using an isocyanate as is disclosed in commonly owned U.S. Pat.
No. 5,641,834, incorporated herein by reference.
Specific examples of adducted hydroxy-terminated polybutadiene include the
reaction of anhydride modified OH-terminal PBD with dibasic anhydride (phthalic
anhydryde), then with an epoxy, such as glycidyl substituents.
A further toughener system utilizes a combination of two polymers having
differing molecular weights as is taught in U.S. Pat. No. 6,225,408. A specific example
taught therein is combination of a major amount of a primary toughener with a weight
average molecular weight (MW) less than about 18,000 together with a minor amount of
an auxiliary toughener with a MW greater than about 18,000. A specific example is a
60:40 mixture of glycidyl methacrylate terminated CTBN rubber, and a terblock
copolymer of styrene-[isoprene]-styrene.
Adhesion promoters useful herein are the known phosphorus-containing
compounds with mono-esters of phosphinic, mono- and diesters of phosphonic and
phosphoric acids having one unit of vinyl or allylic unsaturation present. Vinylic
unsaturation is preferred. Representative of the phoshorus-containing adhesion promoters
are, without limitation, phosphoric acid; 2-methacryloyloxyethyl phosphate; bis-(2-
methacryloxyloxyethyl)phosphate; 2-acryloyloxyethyl phosphate; bis-(2-
acryloyloxyethyl)phosphate; methyl-(2-methacryloyloxyethyl)phosphate; ethyl
methacryloyloxyethyl phosphate; methyl acryloyloxyethyl phosphate; ethyl
acryloyloxyethyl phosphate; propyl acryloylbxyethyl phosphate, isobutyl acryloyloxyethyl
phosphate, ethylhexyl acryloyloxyethyl phosphate, halopropyl acryloyloxyethyl
phosphate, haloisobutyl acryloyloxyethyl phosphate or haloethylhexyl acryloyloxyethyl
phosphate; vinyl phosphonic acid; cyclohexene-3-phosphonic acid; (a-hydroxybutene-2
phosphonic acid; 1 -hydroxy- 1-phenylmethane- 1,1-diphosphonic acid; 1-hydroxy- 1-
methyl-1-disphosphonic acid: 1-amino-1 phenyl-1,1-diphosphonic acid; 3-amino-3-
hydroxypropane-l,l-disphosphonic acid; amino-tris(methylenephosphonic acid); gamma-
amino-propylphosphonic acid; gamma-glycidoxypropylphosphonic acid; phosphoric acid-
mono-2-aminoethyl ester; allyl phosphonic acid; allyl phosphinic acid; ß-
methacryloyloxyethyl phosphinic acid; diallylphosphinic acid; ß-
methacryloyloxyethyl)phosphinic acid and allyl methacryloyloxyethyl phosphinic acid. A
preferred adhesion promoter: is 2-hydroxyethylmethacrylate phosphate.
Another preferred class of adhesion promoters comprises the metal
dimethacrylates. One particularly preferred adhesion promoter comprises zinc
dimethacrylate. These adhesion promoters serve a dual purpose, metallic interaction with
metal surface and crosslinking to strengthen polymer network. In one embodiment of the
present invention, the metal dimethacrylate is present from 0.05 to 4.0 weight percent. In
a preferred embodiment of the present invention, the metal dimethacrylate is present from
about 0.5 to about 2.0 weight percent.
The present invention may also include an ambient temperature initiation system.
The ambient temperature initiation systems that may be employed in the preferred
adhesive systems are well-known redox couple systems and need not be discussed herein
in detail. Basically, such systems comprise at least one oxidizing agent and at least one
reducing agent which are co-reactive at room temperature to generate free radicals
effective to initiate addition polymerization reactions and cure the adhesive. Substantially
any of the known oxidizing and reducing agents which are so co-reactive can be
employed. Representative oxidizing agents include, without limitation, organic peroxides,
such as benzoyl peroxide and other diacyl peroxides, hydroperoxides such as cumene
hydroperoxide, peresters such as p-butylperoxybenzoate; ketone hydroperoxides such as
methyl ethyl ketone hydroperoxide, organic salts of transition metals such as cobalt
naphthenate, and compounds containing a labile chlorine such as sulfonyl chloride.
Representative reducing agents include, without limitation, sulfmic acids; azo compounds
such as azoisobutyric acid dinitrile; alpha-aminosulfones such as bis(tolysulfonmethyl)-
benzyl amine; tertiary amines such as diisopropanol-p-toluidine (DIIPT), dimethyl aniline,
p-halogenated aniline derivatives and dimethyl-p-toluidine; and aminealdehyde
condensation products, for example, the condensation products of aliphatic aldehydes such
as butyraldehyde with primary amines such as aniline or butylamine. The use of known
accelerators and promoters with the redox couple catalyst systems can be advantageous.
Preferably, the oxidizing agent will be present in an amount in the range from about 0.5 to
about 50 percent by weight of bonding accelerator, with the amount of reducing agent
being in the range from about 0.05 to about 10 preferably about 0.1 to about 6.0 percent by
weight of polymerizable adhesive composition. DUPT is the most preferred reducing
agent. The most preferred oxidizing agent is benzoyl peroxide.
Although the adhesive of the present invention may take many forms, the most
preferred adhesive systems are provided as multipack or two-part adhesive systems where
one package or part contains the free radical-polymerizable monomer component and the
reducing agent and a second part or package contains the oxidizing agent. The two parts
are mixed together at the time of use in order to initiate the reactive cure. After mixing the
individual parts, one or both surfaces to be joined are coated with the mixed adhesive
system and the surfaces are placed in contact with each other.
Other optional additives which are typically considered in fully formulated
adhesives include antioxidants, inhibitors, anti-sag additives, thixotropes, processing aids,
waxes, UV stabilizers, arc suppressants, and drip suppressants. Examples of typical
additives are fumed silica, alumina, hindered phenols, substituted hydroquinone, silane-
treated talc, mica, feldspar, and wollastonite.
USES
Although the adhesive of the present invention may take many forms, the most
preferred adhesive systems are provided as multipack or two-part adhesive systems where
one package or part contains the polymerizable or reactive components and the reducing
agent and a second package or part contains the oxidizing agent. The two parts are mixed
together at the time of use in order to initiate the reactive cure. The preferred means for
dispensing the adhesive are two-chambered cartridges equipped with static mixers in the
nozzle, and for larger scale application, meter mix dispensing equipment. After mixing the
individual packages, one or both surfaces to be joined are coated with the mixed adhesive
system and the surfaces are placed in contact with each other. Preferred mix ratios
typically include from 1:1 to 10:1 of A:B.
The adhesive systems of the invention may be used to bond metal surfaces, such as
steel, aluminum and copper, to a variety of substrates, including metal, plastics, and other
polymers, reinforced plastics, fibers, glass, ceramics, wood and the like. The adhesives
. are particularly useful in hem flange bonding of auto body panels. It is a feature of the
present invention that the herein-described adhesive compositions can be employed to
bond metal substrates such as steel, aluminum and copper with little, if any, pretreatment
of the metal surface prior to application of the adhesive. Thus, bonding can be effected
even to oily metal surfaces which are otherwise clean without an extensive pretreatment as
is usually required with the vast majority of currently available primers and adhesives.
Additionally, the adhesive systems of this invention provide effective bond strength at
room temperature, thus heat is not required either for applying the adhesive systems to the
substrates or for developing handling strength and dimensional stability.
Although the adhesives of the present invention are preferred for bonding metal
surfaces, the present adhesive compositions may be applied as an adhesive, primer or
coating to any surface or substrate capable of receiving the adhesive. The metals which are
preferred for bonding with the present adhesives include zinc, copper, cadmium, iron, tin,
aluminum, silver, chromium, alloys of such metals, and metallic coatings or platings of
such metals such as galvanized steel including hot dipped, electrogalvanized steel and
galvanealed steel.
The adhesive may be brushed, rolled, sprayed, dotted, knifed, cartridge-applied,
especially from a dual cartridge; or otherwise applied to one substrate, but preferably to
both substrates to desired thickness preferably not to exceed 60 mils. The substrates may
be clamped for firmness during cure in those installations where relative movement of the
two substrates might be expected. For example, to adhere metal surfaces, an adherent
quantity of the adhesive composition is applied to one surface, preferably to both surfaces,
and the surfaces are confronted with the adhesive composition therebetween. The adhesive
should have a thickness less than 60 mils for optimum results. The smoothness of the
surfaces and their clearance (e.g., in the case of nuts and bolts) will determine the required
film thickness for optimum bonding.
The two metal surfaces and the interposed adhesive composition are maintained in
engagement until the said adhesive composition has cured sufficiently to bond the said
surfaces. Cure advancement may be promoted by post-baking the bonded parts after an
initial cure time at room temperature. The post-baking preferably takes place above about
150 °C. and below about 190 °C. Additionally, incorporation of glass beads to control
bondline thickness is preferred especially in hemming operations, as is taught in U.S. Pat.
Nos. 5,487,803 and 5,470,416.
Although the present invention has been described with reference to particular
embodiments, it should be recognized that these embodiments are merely illustrative of
the principles of the present invention. Those of ordinary skill in the art will appreciate
that the compositions, apparatus and methods of the present invention may be constructed
and implemented in other ways and embodiments. Accordingly, the description herein
should not be read as limiting the present invention, as other embodiments also fall within
the scope of the present invention as defined by the appended claims.
Method of Manufacturing the A side composition
1. Add the primary monomer to a vessel and heat while stirring to no more than 60
°C.
2. Next, slowly add the maleimide terminated polyimide (MTP).
3. Maintain the temperature until there are no visible particles of MTP.
4. Continue heat and mixing for an additional hour to ensure complete extension of
MTP molecule.
5. Allow the solution to cool to approximately 40°C, then add the remaining
constituents.
* The MTP resin employed in these examples is a bis-maleimide terminated polyimide
hereinafter referred to as "MTP".
The composition is them mixed in a 10:1 A:B ratio and applied to substrates for
testing. Both Impact Wedge Peel (IWP) and Lap Shear Strength (LSS) of the adhesive
were tested as is known in the art. The key experimental results are listed below evidence
the superior performance of MTP-modified acrylic formulations over commercially
successful prior art systems.
Failure modes are described as follows:
¦ c - cohesive failure; bond cleavage takes place through adhesive bulk; a layer
adhesive deposits on both substrates with even thickness
¦ tlc - thin layer cohesive failure; bond cleavage still takes place through adhesive
bulk; a thinner layer of adhesive deposits on one substrate and a thicker layer on the other
¦ a - adhesive failure: bond cleavage takes place at interface of substrate and
adhesive, leaving bare substrate on one or both substrates without adhesive deposit
Approximately 100% cohesive failure (with small portion of thin layer cohesive
failure) was achieved on both aluminum (6061 T6, 0.030", ACT) and electro-galvanized
steel (EGS) (0.030", ACT).
Samples P1 and P3 were tested against a common commercially successful
structural adhesive, Versilok® adhesive, available from LORD Corporation, Cary, NC,
USA, hereinafter "PA Adhesive"

1) The MTP-modified formulation demonstrated stronger lap shear strength at under
bake (160 °C/15 min.), normal bake (175 °C/25 min.), and over-bake (190 °C/45 min.)
conditions; neither under or over-bake compromised performance, showing the robustness
of the system.
Under and over-bake lap shear strength (PSI) performance evaluation:
4) Prototype consistently showed higher Impact Wedge Peel strength (toughness) than the
PA Adhesive on oiled metal at 22 °C. and 80 °C. testing temperatures.
WE CLAIM
1. A structural adhesive comprising:
a free radical-polymerizeable monomer; and,
a maleimide terminated polyimide wherein the maleimide comprises a mono-, bis-,
or poly-maleimide compound.
2. The composition of claim 1, wherein the radical-polymerizable monomer
comprises THFMA.
3. The composition of claim 1, further comprising a toughener.
4. The composition of claim 3, wherein the toughener comprises at least one of
GMA/CTB adducts, core-shell impact modifiers, and block copolymer elastomers.
5. The composition of claim 1, further comprising an adhesion promoter.
6. The composition of claim 5, wherein the adhesion promoter comprises HEMA
phosphate.
7. The composition of claim 1, further comprising a metal dimethacrylate.
8. The composition of claim 7, wherein the metal dimethacrylate comprises zinc
dimethacrylate.
9. The composition of claim 1, further comprising ethoxylated bisphenol A
dimethacrylate.
10. The composition of claim 1, further comprising a particulate additive comprising at
least one of calcium metasilicate or fumed silica.
11. A two part structural adhesive composition comprising:
in part A: (a) at least one free radical-polymerizable monomer; (b) a maleimide
terminated polyimide; and (c) a reducing agent; and
in part B: an oxidizing agent.
12. An adhesive according to claim 11, wherein the reducing agent comprises at least
one of N,N-diisopropanol-p-chloroaniline; N,N-diisopropanol-p-bromoaniline; N,N-
diisopropanol-p-bromo-m-methylaniline; N.N-dimethyl-p-chloroaniline; N,N-dimethyl-p-
bromoaniline; N,N-diethyl-p-chloroaniline; N,N-diethyl-p-bromoaniline; N,N-dimethyl-p-
aniline; and N,N-diisopropanol-p-toluidine.
13. The adhesive of claim 11, wherein the oxidizing agent comprises benzoylperoxide.
14. The adhesive composition of claim 11, wherein the weight ratio of the first
package to the second package is from about 1:1 to about 15:1.
15. A method for bonding two substrates comprising:
providing a structural adhesive composition comprising a maleimide terminated
polyimide on a first substrate; and
contacting the second substrate to the structural adhesive on the first substrate and
curing the structural adhesive.
16. The method of claim 15 wherein the structural adhesive comprises at least one of
an acrylic-based structural adhesive or an epoxy-based structural adhesive.

A maleimide terminated polyimide incorporated into a two-part acrylic structural
adhesive system. The maleimide terminated polyimide of the various embodiments
of the present invention provide improving thermal stability, strength, and
toughness.