AMINE CO-ACCELERATOR FOR ACRYLIC ADHESIVES
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority under 35 U.S. C. § 119(e) from U.S.
Provisional Patent Application Serial No. 61/955,251 entitled "AMINE COACCELERATOR
FOR ACRYLIC ADHESIVES", filed March 19, 2014, the disclosure of
which is herein incorporated by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to co-accelerators for free radical cured acrylic
adhesives that enhance the activity of the reducing agent. These co-accelerators deliver fast
cure speed of the adhesive with surprisingly long open time, render the adhesive cure rate
insensitive to acidic adhesion promoters, and allow the acrylic adhesive to bond strongly to a
variety of metals, including aluminum.
BACKGROUND OF THE INVENTION
[0003] 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.
[0004] Acrylic structural adhesives are extensively used for providing structural strengthimparting
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
[0005] Conventional acrylic structural adhesives typically comprise a mixture of one or more
olefinic reactive monomers such as methylmethacrylate and methacrylic acid, toughener(s)
and redox initiator system. The toughener(s) 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.
SUMMARY OF THE INVENTION
[0006] In an embodiment of the present invention, a curable composition is provided
comprising a free-radical polymerizable acrylic monomer, an oxidizing agent, an aromatic
tertiary amine reducing agent, and a tertiary amine co-accelerator comprising a bicyclic diaza
compound that increases the reaction rate between the oxidizing agent and the reducing
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,Ndiisopropanol-
p-bromo-m-methylaniline; N,N-dimethyl-p-chloroaniline; N,N-dimethyl-pbromoaniline;
N,N-diethyl-p-chloroaniline; N,N-diethyl-p-bromoaniline; N,N-dimethyl-paniline;
or N,N-diisopropanol-p-toluidine, and most preferably N,N-diisopropanol-paratoluidine.
[0007] In another embodiment of the present invention, the co-accelerator comprises 1,4-
diazabicyclo[2.2.2]octane. In a further preferred embodiment of the present invention, the
tertiary amine reducing agent comprises N-N-diisopropanol-para-toluidine and the coaccelerator
comprises l,4-diazabicyclo[2.2.2]octane and the composition is otherwise free of
amine compounds.
[0008] In one embodiment of the present invention, the composition is free of isocyanate
compounds. In a further embodiment of the present invention, the free radicalpolymerizeable
monomer comprises at least one of methylmethacrylate, or tetrahydrofurfuryl
methacrylate.
[0009] In another embodiment of the present invention, the composition further comprises a
toughener, preferably at least one of glycidyl methacrylate/carboxyl terminated butadiene
(GMA/CTB) adducts, core-shell impact modifiers, or block copolymer elastomers. In one
embodiment of the present invention, the composition further comprises an adhesion
promoter, preferably at least one of hydroxyethyl methacrylate phosphate or methacrylic
acid. In another preferred embodiment of the present invention, the oxidizing agent
comprises benzoyl peroxide.
[0010] In an additional embodiment of the present invention, the composition is provided in
two parts: in part A: (a) the at least one free radical -polymerizable monomer; and (b) the
reducing agent; and in part B: an oxidizing agent; wherein the co-accelerator is present in at
least one of part A or part B; preferably the weight ratio of part A to part B is from about 1:1
to about 15:1.
[0011] In yet another embodiment of the present invention, the composition is disposed
between two substrates and cured so as to provide a bond therebetween having a bond
strength of at least lOOOpsi as measured by lap shear strength. In one embodiment of the
present invention, at least one of the two substrates comprises aluminum.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 represents the ideal acrylic adhesive cure profile evidencing reaction
propagation of a "snap cure" temperature profile.
[0013] FIG. 2 illustrates the temperature profile of curing compositions of the prior art as
compared to embodiments of the present invention.
[0014] FIG. 3 illustrates the temperature profile of curing compositions of the prior art as
compared to embodiments of the present invention.
[0015] FIG 4. Illustrates ROBDS and DMOT data of curing compositions of the prior art as
compared to embodiments of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0016] In a first aspect of the present invention, an acrylic adhesive composition is provided
comprising a free-radical polymerizable monomer, an initiation system and a co-accelerator
that enhances the reactivity of the initiation system by increasing the reaction rate between
the oxidizing agent and the reducing agent. In a preferred embodiment of the present
invention, the free radical-polymerizable monomer comprises methyl methacrylate (MMA),
the initiator system comprises N,N-diisopropanol-p-toluidine (DIIPT) and benzoyl peroxide
(BPO), and the co-accelerator l,4-diazabicyclo[2.2.2]octane (or triethylene diamine (TDA)).
[0017] In one embodiment of the present invent, the acrylic adhesive comprises 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, 2-ethylhexyl 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.
[0018] 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, or diacids such as itaconic acid, maleic acid and
fumaric acid. Further optional co-monomers 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-l,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.
[0019] Co-monomers optionally includable with the primary monomer include OHfunctional
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. Preferably 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.
[0020] The present invention includes 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 coreactive
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 b-
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. In one preferred embodiment of the present invent,
the oxidizing agent comprises an organic peroxide, most preferably benzoyl peroxide (BPO).
[0021] In another preferred embodiment of the present invention, the reducing agent
comprises an aromatic tertiary amine. Representatives reducing agents include at least one of
N,N-diisopropanol-p-chloroaniline; N,N-diisopropanol-p-bromoaniline; N,N-diisopropanolp-
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; N,Ndimethyl-
p-toluidine; N,N-diethyl-p-toluidine; N,N-diisopropanol-p-toluidine (DIIPT), or
other p-halogenated aniline derivatives.
[0022] In one embodiment of the present invention, the reducing agent is present 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 and the oxidizing agent is present in an amount in the
range from about 0.5 to about 50 percent by weight of reducing agent. In a most preferred
embodiment of the present invention, the reducing agent comprises DIIPT and the oxidizing
agent comprises benzoyl peroxide.
[0023] In a further embodiment of the present invention, the acrylic adhesive composition
comprises a co-accelerator. The co-accelerator is distinct from the accelerator/reducing agent
as it will not accelerate the decomposition of the oxidizing agent, and therefore will not
facilitate cure of the composition alone. However, when employed with a reducing agent as
mentioned above, the co-accelerator will enhance the reactivity of the system providing more
of a "snap cure" than the reducing agent alone.
[0024] In a preferred embodiment of the present invention, the co-accelerator comprises a
cyclic tertiary amine, and preferably a tertiary amine comprising a bicyclic diaza compound
wherein the nitrogen atoms are present in the rings at the juncture of the rings. In a most
preferred embodiment of the present invention, the co-accelerator comprises 1,4-
diazabicyclo[2.2.2]octane (TDA). In an embodiment of the present invention, the coaccelerator
is present in an amount from 0.10 to 3.0 weight percent based on the total weight
of the composition.
[0025] Though not wishing to be bound by the theory, co-accelerators of the present
invention are thought to participate in the reaction to enhance the action of the reducing
agent, without directly facilitating the decomposition of the oxidizer. The mechanics are
believed to be driven by the higher basicity of the cyclic tertiary amine, which complexes
with acidic components of the composition to enhance the reactivity of the reducing agent
and oxidizer. Further, the co-accelerators of the present invention will not react with the
oxidizer and cure the composition by themselves, and must be employed with a primary
reducing agent as described above.
[0026] In one preferred embodiment of the present invention, the composition is preferably
free of isocyanate functionality, i.e. reactive NCO groups. At times, isocyanates and/or
polyurethanes are added to acrylic structural adhesives, however in this embodiment of the
present invention a composition essentially free of isocyanate functionality is preferred. In a
most preferred embodiment of the present invention, the composition is completely free of
isocyanate reactive components.
[0027] In an additional embodiment of the present invention the acrylic composition further
comprises a toughener. Examples of suitable tougheners 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
olefinic 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.
[0028] 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.
[0029] 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. In one preferred embodiment of the present invention, the toughener is based on a
terblock polymer of styrene- [isoprene] -styrene, 25-[50] -25, parts by weight. Additional
commercial block copolymers comprise the Kraton® family available from Kraton Polymers,
Inc, such as the Kraton SBS and SIS family of copolymers. 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. In another preferred embodiment of the present invention, the toughener
comprises a styrene-butadiene-styrene block copolymer.
[0030] 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 homopolymers and/or copolymers.
[0031] 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.
[0032] Specific examples of adducted hydroxy-terminated polybutadiene include the reaction
of anhydride modified OH-terminal PBD with dibasic anhydride (phthalic anhydride), then
with an epoxy, such as glycidyl substituents.
[0033] 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.
[0034] In an additional embodiment of the present invention and adhesion promoter is added
to the acrylic composition. Adhesion promoters useful herein are the known phosphoruscontaining
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 phosphorus-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 acryloyloxyethyl 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-
1phenyl- 1,1-diphosphonic acid; 3-amino-3-hydroxypropane- 1,1 -disphosphonic acid; aminotris(
methylenephosphonic acid); gamma-amino-propylphosphonic acid; gammaglycidoxypropylphosphonic
acid; phosphoric acid-mono-2-aminoethyl ester; allyl phosphonic
acid; allyl phosphinic acid; b-methacryloyloxyethyl phosphinic acid; diallylphosphinic acid;
-methacryloyloxyethyl)phosphinic acid and allyl methacryloyloxyethyl phosphinic acid. A
preferred adhesion promoter: is 2-hydroxyethylmethacrylate phosphate (HEMA-phosphate).
[0035] In a further embodiment of the present invention, the adhesion promoter comprises
acids with acrylate functionality, including acrylic acid, methacrylic acid, crotonic acid,
isocrotonic acid, fumaric acid, maleic acid, cinnanic acid, 2-methylmaleic acid, itaconic acid,
2-methylitaconic acid, sorbic acid, and a-b-methylene glutaric acid.
[0036] 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.
[0037] 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.
[0038] 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 15:1, and more preferably 4:1 to 10:1 of A:B.
[0039] 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. 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.
[0040] 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.
[0041] 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.
[0042] The two metal surfaces and the interposed adhesive composition are maintained in
engagement until the adhesive composition has cured sufficiently to bond the surfaces. Cure
advancement may be promoted by post-baking the bonded parts after an initial cure time at
room temperature. 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.
[0043] 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.
EXAMPLES
[0044] Example 1 - Cure Rate
Cure rate for acrylic adhesives is measured through Test Method 15 (TM-15). In this
test, approximately 20 grams of mixed adhesive preconditioned at 25 °C was monitored for
the time (after mixing) required in order to reach a maximum temperature generated by the
reaction exotherm. The maximum temperature reached is also recorded. This allows for a
relatively simple comparison of the rate of reaction (initiation and propagation) between
different adhesive formulations. Additional information can be gained through closer
examination of the temperature change over time. In looking at the rate of temperature
change through the curing process, particularly the amount of time spent with minimal
temperature rise versus the rate of temperature rise through the cure peak, one can estimate
the ratio of Open Time vs Time to Handling Strength (described in more detail below). In the
ideal reaction scenario, there is no rise in temperature for a period of time as the inhibitors
block the polymerization reaction propagation, and then an instantaneous rise to the
maximum temperature as the adhesive cures very rapidly and delivers high bond strength, as
shown below. This is commonly referred to as "snap" cure and can be seen in Fig. 1.
[0045] Initial experiments were performed in order to compare the effect of various amine
accelerators on the TM-15 cure speed of the chosen model acrylic adhesive system. A
masterbatch formulation was prepared without amines according to the recipe in Table 1.
Table 1 - Masterbatch
[0046] Reducing agents and co-accelerators were then added to form comparative samples
that could be cured using Lord Accelerator 17 (Commercially available from LORD
Corporation and employing a BPO based oxidizer), with the amounts shown in Table 2 below
added to 98.2 grams of Masterbatch.
Table 2 - Reducing agent & co-accelerator
[0047] Sample 1 contains the reducing agent N,N-diisopropanol-p-toluidine (DIIPT) with no
co-accelerator. Sample 2 contains DIIPT with anther reducing agent N,N-dimethyl-p-aniline
(DMA). Sample 3 contains the reducing agent DIIPT with a common tertiary amine additive
dimethylpiperazine (DMP). Sample 4 contains the reducing agent DIIPT with the inventive
co-accelerator l,4-diazabicyclo[2.2.2]octane (TDA). For samples 1 through 4, direct
comparison was made of equal weights of amine compounds, i.e. reducing agent and
accelerator combined. TM-15 plots are shown in Fig. 2.
[0048] Within this data set, and illustrated in Fig. 2 it can be seen that the TDA containing
formulation delivers much higher cure speed at the same weight loading compared to either
DIIPT alone or DIIPT plus DMA, and slightly better than the DMP containing formulation.
Sample 2A was an attempt to adjust the amount of the DIIPT/DMA accelerator system to
match the cure speed of DIIPT/TDA for further experiments, since it is important for a fair
comparison to evaluate the properties of acrylic adhesives that have similar cure speeds.
Note that it was not possible to adjust the cure speed of the formulation containing only
DIIPT so that the cure speed would approach 15 minutes. The TM-15 plots of the rateadjusted
samples are shown in Fig. 3.
[0049] It can be seen in this data set in Fig. 3 that even with a significant increase of DMA
loading (above the molar equivalence to TDA) and concurrent increase in DIIPT loading to
increase the cure speed, the rate of temperature rise does not have the same characteristics of
"snap" cure as the TDA containing sample. The earlier rise in temperature and dramatically
lower peak exotherm for sample 2A show that the reaction kinetics are significantly different.
As mentioned above, the amine accelerator package must overcome the cure inhibition of
both the added inhibitors and the acidic adhesion promoters. It is believed that the generation
of stronger "snap" cure by TDA is due to the stronger basicity that forms a strong complex
with the acids and eliminates this inhibition to the cure rate. This is also observed to a
slightly lesser extent in the DMP containing formulation, however the monocyclic structure
of DMP is believed to hinder the exceptional performance exhibited by TDA. It will be seen
that these cure characteristics will be reflected in the Open Time and Time to Handling
Strength discussed below.
[0050] Example 2 - Open Time vs. Time to Handling Strength
Open Time is defined as the time allowed after the adhesive bead has been applied
and before the substrates are mated that will deliver essentially equivalent bond strength as if
the substrates were mated upon initial application of the adhesive. 'Essentially equivalent"
bond strength is commonly understood to be within about 10% of the bond strength achieved
with immediate mating of the substrates. Time to Handling Strength is defined as the time
required after the adhesive bead has been applied and mated for the bond strength to exceed
100 psi. For acrylic adhesives, a longer Open Time will typically result in a longer Time to
Handling Strength. It is generally most desirable to have a Time to Handling Strength that is
minimally longer than the Open Time, referred to as "snap" cure or sometimes "cure on
command". Typically, acrylic adhesives exhibit final bond strength of between 2,000-
3,000psi.
[0051] Open Time is measured through a test called Delayed Mating Open Time (DMOT).
In this test, adhesive is applied to a series of 1" x 4" aluminum coupons as quickly as
possible, then coupons are mated with a delay of successively longer intervals of time, with
0.5" overlap between the two coupons and a 10 mil bond line thickness. The adhesive is then
allowed to cure completely (typically overnight), and the coupons are pulled apart in shear.
The results of Lap Shear Strength (LSS) and Failure Mode (adhesive vs. cohesive) are
compared for each coupon to one mated with effectively zero delay, and the end of the Open
Time is then judged by a reduction in strength of >10% of the original and/or reduction in
cohesive failure to <80% (>20% adhesive failure).
[0052] Time to Handling Strength is measured through a test called Rate of Bond Strength
Development (ROBSD). In this test, adhesive is applied to a series of 1" x 4" aluminum
coupons which are then immediately mated to a second coupon with 0.5" overlap and a 10
mil bond line thickness. The coupons are then pulled apart in shear over successively longer
intervals of time, evaluating the Lap Shear Strength over time as the adhesive cures and bond
strength increases. The Time to Handling Strength is judged by the time that it takes for the
bond strength to reach 100 psi.
[0053] Data for the ROBSD and DMOT testing are shown in the table below and in Fig. 4.
As mentioned before, it is most relevant to compare acrylic adhesive systems that have
roughly the same TM-15 cure rate. The ROBSD data in the table directly shows the faster
reaction kinetics with the use of TDA as a co-accelerator. Note that the ROBSD time is
significantly longer than the TM-15 time because the thin bond line and metal coupons
greatly reduce the generation of heat during reaction. Thus these curing reactions occur at
much closer to a constant (and lower) temperature than in the TM-15 test. The ratio of
ROBSD to DMOT clearly shows a much more desirable "snap" cure with the use of TDA,
especially compared to the sample containing DMA (exhibiting the lowest ROBSD/DMOT
ratio), and highlights the improvement over the current reaction enhancer DMP.
[0054] Example 3 - Cure rate sensitivity to acidic compounds
Adhesion promoters for acrylic adhesives typically include acidic compounds which
are known to facilitate bonding to metal. One preferred adhesion promoter is 2-
hydroxyethylmethacrylate phosphate (HEMA-phosphate). However, HEMA-phosphate is
known to slow the cure speed of DIIPT cured adhesives and as such there exists a trade-off
between adhesion and cure rate. For example, in the commercially available Lord 410/19
using a combination of DIIPT and N,N-dimethylaniline as the reducing agent and BPO as the
oxidizer, the addition of about five hundredths of one percent HEMA-Phosphate will slow the
Time to Peak Exotherm of a 20 gram mass by one minute. Surprisingly, the combination of
DIIPT with TDA causes the formulation to be insensitive to even large changes in HEMAPhosphate
loading allowing for the use of greater quantities of adhesion promoter without
changing the cure rate. For example, in an experimental formulation containing DIIPT and
TDA, the addition of 3% HEMA-Phosphate slowed the Time to Peak Exotherm of a 20 gram
mass by only 12 seconds (essentially within error of the test).

CLAIMS
What is claimed is:
1. A curable composition comprising a free-radical polymerizable acrylic monomer, an
oxidizing agent, an aromatic tertiary amine reducing agent, and a tertiary amine coaccelerator
comprising a bicyclic diaza compound that increases the reaction rate between the
oxidizing agent and the reducing agent.
2. The composition of claim 1, wherein the reducing agent comprises at least one of
N,N-diisopropanol-p-chloroaniline; N,N-diisopropanol-p-bromoaniline; N,N-diisopropanolp-
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; or N,Ndiisopropanol-
p-toluidine.
3. The composition of claim 2, wherein the aromatic tertiary amine reducing agent
comprises N,N-diisopropanol-para-toluidine.
4. The composition of claim 1, wherein the co-accelerator comprises 1,4-
diazabicyclo [2.2.2]octane.
5. The composition of claim 1, wherein the tertiary amine reducing agent comprises NN-
diisopropanol-para-toluidine and the co-accelerator comprises 1,4-
diazabicyclo[2.2.2]octane and the composition is otherwise free of amine compounds.
6. The composition of claim 1, wherein the composition is free of isocyanate
compounds.
7. The composition of claim 1, wherein the free radical-polymerizeable monomer
comprises at least one of methylmethacrylate, or tetrahydrofurfuryl methacrylate.
8. The composition of claim 1, further comprising a toughener.
9. The composition of claim 8, wherein the toughener comprises at least one of glycidyl
methacrylate/carboxyl terminated butadiene (GMA/CTB) adducts, core-shell impact
modifiers, or block copolymer elastomers.
10. The composition of claim 1, further comprising an adhesion promoter.
11. The composition of claim 10, wherein the adhesion promoter comprises hydroxyethyl
methacrylate phosphate.
12. The composition of claim 1, further comprising methacrylic acid.
13. The composition of claim 1, wherein the oxidizing agent comprises benzoyl peroxide.
14. The composition of claim 1, wherein the composition is provided in two parts: in part
A: (a) the at least one free radical-polymerizable monomer; and (b) the reducing agent; and in
part B: an oxidizing agent; wherein the co-accelerator is present in at least one of part A or
part B.
15. The composition of claim 14, wherein the weight ratio of part A to part B is from
about 1:1 to about 15:1.
16. The composition of claim 1 disposed between two substrates and cured so as to
provide a bond therebetween having a bond strength of at least lOOOpsi as measured by lap
shear strength.
17. The composition of claim 16 wherein at least one of the two substrates comprises
aluminum.