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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a thermosetting adhesive based on a reactive
acrylate and/or methacrylate copolymer.
2. Description of the Prior Art
German Pat. No. 25 24 197 discloses adhesives composed of a terpolymer,
consisting of
20 to 45 weight percent of acrylonitrile and/or methacrylonitrile,
45 to 70 weight percent of an alkyl acrylate with 1 to 12 carbon atoms in
the alkyl radical and
1 to 10 weight percent of acrylic, methacrylic or itaconic acid, wherein
the sum of the monomers amounting to 100 weight percent and the copolymer
obtained having a limiting viscosity of 0.1 to 0.8 (100
ml.multidot.g.sup.-1) at 20.degree. C. in chloroform, optionally in
admixture with up to 35 weight percent of an epoxide resin or a
phenol-formaldehyde resin containing glycidyl groups.
These adhesives make glue bonds of high tensile shear strength and good
peel strength possible. The adhesive can be used as films for gluing metal
surfaces and also for gluing plastic film to metal surfaces. If these
adhesives do not contain epoxide resins or phenol-formaldehyde resins
containing glycidyl groups, they remain thermoplastic. In the presence of
epoxide resins or phenol-formaldehyde resins containing glycidyl groups,
the adhesives are thermosetting with temperatures of about 130.degree. C.
being required for the curing.
In German patent application No. P 29 26 284.8-43, which has not yet been
published, a thermosetting adhesive is described which contains the
following active components:
(a) 20 to 80 weight percent of a copolymer with was obtained by the
polymerization of
(a.sub.1) 20 to 45 weight percent of acrylonitrile and/or
methacrylonitrile,
(a.sub.2) 35 to 65 weight percent of one or several alkyl esters of acrylic
and/or methacrylic acid with 1 to 12 carbon atoms in the alkyl radical,
(a.sub.3) 10 to 20 weight percent of acrylic, methacrylic and/or itaconic
acid,
(a.sub.4) 0 to 15 weight percent of acrylamide and/or methacrylamide,
the sum of the monomers amounting to 100 weight percent and the copolymer
having a limiting viscosity of 0.1 to 0.8 (100 ml.multidot.g.sup.-1) at
20.degree. C. in methyl acetate,
(b) 10 to 70 weight percent of an epoxide resin and/or a
phenol-formaldehyde resin containing glycidyl groups,
(c) 0.2 to 15 weight percent of a hardener for the components (b), which is
effective at elevated temperatures,
(d) 0 to 5 weight percent of an accelerator for the reaction of components
(b) with components (a) and (c),
the sum of (a) to (d) amounting to 100 weight percent, and optionally the
usual additives, such as, gluing auxiliaries, adhesion promoters, pigments
and fillers.
Compared to the adhesives of German Pat. No. 25 24 197, these adhesives
cure at temperatures of 100.degree. C. to 130.degree. C. Consequently, it
is possible to keep the thermal stress on the plastics to be glued at a
lower level and to retain desirable properties, such as, the surface
quality and gloss of the plastics. The composite materials of plastic and
metal, obtained by the gluing, can be molded in the usual manner without
impairing the adhesion.
SUMMARY OF THE INVENTION
We have discovered an adhesive which can be cured reactively at lower
temperatures, for example, already at 80.degree. C. and below. As a
result, such adhesives have wider applicability than the higher curing
materials, especially in gluing plastics. They can be used in the
manufacture of skis based on composite materials, since temperatures of
90.degree. C., for example, are not usually exceeded in the manufacture of
skis. However, this lowering of the curing temperature cannot be achieved
at the expense of a reduction in the adhesion to the contacting surfaces.
With the present invention, the adhesive and elastic properties, as well
as the resistance to moisture and other weathering effects are retained.
More particularly, the adhesive composition of the present invention
contains as the active components:
(a) a copolymer, which is obtained by the polymerization of
(a.sub.1) 35 to 70 weight percent of one or several alkyl esters of acrylic
and/or methacrylic acid with 1 to 8 carbon atoms in the alkyl radical,
(a.sub.2) 20 to 45 weight percent of acrylonitrile and/or methacrylonitrile
and/or vinyl acetate,
(a.sub.3) 10 to 20 weight percent of one or several .omega.-hydroxyalkyl
esters of acrylic and/or methacrylic acid with 1 to 5 carbon atoms in the
alkyl radical,
(a.sub.4) 0 to 15 weight percent of acrylamide and/or methacrylamide, and
(a.sub.5) 0 to 35 weight percent of acrylic or vinyl monomers, whose
composition is different from that of the components of (a.sub.1) to
(a.sub.4),
the sum of the monomers (a.sub.1) to (a.sub.5) amounts to 100 weight
percent and the copolymer has a limiting viscosity of 0.05 to 0.6 (100
ml.multidot.g.sup.-1) at 25.degree. C. in methyl acetate; and
(b) one or more polyisocyanates and/or their partial reaction products with
polyols, the molecule having at least 2 isocyanate groups on the average,
wherein
components (a) and (b) being present in a ratio such that one hydroxyl
group of component (a) corresponds to 1 to 1.5 isocyanate groups of
component (b). The composition may also optionally contain conventional
additives, such as, accelerators, gluing auxiliaries, pigments and
fillers.
The principle on which the curing reaction of the adhesive of the present
invention is based is the reaction between the hydroxyl groups of the
.omega.-hydroxyalkyl esters of acrylic and/or methacrylic acid (component
a.sub.3) and the isocyanate groups of the polyisocyanates or their partial
reaction products with polyols (component b). However, in practice, with
this reaction principle, the adhesion of the adhesives to the surface of
plastics is generally unsatisfactory when polyethers are used as the
polyol component. When polyester polyols are used, the adhesion to metal
surfaces is not permanently resistant to the action of water or water
vapor. However, with the composition of the present invention, all of
these desired properties are present.
BRIEF DESCRIPTION OF THE DRAWING
The FIGURE is a graph of the drum peel strength variation with limiting
viscosity.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The preferred acrylate and/or methacrylate copolymer used in the present
invention has a limiting viscosity of 0.1 to 0.4 (100
ml.multidot.g.sup.-1) at 25.degree. C. in methyl acetate.
The polymerization of the copolymer (a) is accomplished by conventional
procedures known to those skilled in the art. The polymerization can be
carried out as an emulsion or a solution polymerization. The
polymerization in a solvent is preferred, because, in view of the reaction
with isocyanates, careful drying of the products of an emulsion
polymerization is required.
Examples of suitable monomers (a.sub.1) are ethyl acrylate, butyl acrylate,
butyl methacrylate, methyl acrylate, methyl methacrylate,
2-ethylhexylacrylate and 2-ethylhexylmethacrylate.
Examples of suitable monomers (a.sub.3) are 2-hydroxyethyl acrylate or
-methacrylate, 3-hydroxypropylacrylate or -methacrylate,
1,4-butanediol-monoacrylate and 1,4-butanediol-monomethacrylate.
Examples of suitable monomers (a.sub.5) are styrene, vinyl propionate and
methyl vinyl ketone.
As the polyisocyanate, a polyisocyanate of the general formula
##STR1##
in which n.ltoreq.3, or
##STR2##
is preferably used. These isocyanates are commercially available. However,
other polyisocyanates, such as, for example, 1,6-hexamethylendiisocyanate,
2,4,4-trimethyl-1,6-hexamethylendiisocyanate,
3-isocyanatomethyl-3,5,5-trimetylcyclohexylisocyanate,
1,4-tetramethylendiisocyanate, 2,4- and 2,6-hexahydrotoluylene
diisocyanate, hexahydro-1,3- or -1,4-phenylene-diisocyanate, 1,3- and
1,4-phenylenediisocyanate, 2,4- and 2,6-toluylene diisocyanate,
4,4'-diisocyanato-diphenylmethane, naphthylene-1,5-diisocyanate,
m-xylylene-diisocyanate, tris-(4-isocyanatophenyl)-thiophosphate,
4,4',4"-triisocyanato-triphenylmethane, 2,4,6-triisocyanato-toluene or
2,4,4'-triisocyanato-diphenylether are suitable. The isocyanates must,
however, fulfill the condition that they have on the average at least 2
isocyanate groups per molecule.
Instead of the pure polyisocyanates, their partial reaction product with
polyether or polyester polyols can also be used.
In this connection, an adhesive is preferred in which component (b) is a
partial reaction product of one or several polyisocyanates with polyether
or polyester polyols having a molecular weight less than 2000, and having
on the average at least 2 hydroxyl groups in the molecule. The
polyisocyanates are reacted with the polyols in a ratio such that one
hydroxyl group corresponds to 1.5 to 3 isocyanate groups. Examples of
suitable polyether polyols are linear or branched polyether polyols based
on ethylene oxide, propylene oxide and butylene oxide, polythioethers and
adducts of ethylene oxides on polyamines and alkoxylated phosphoric acids.
Examples of suitable polyester polyols are linear or branched polyester
polyols, such as, those obtained in the usual manner from multifunctional
and preferably bifunctional carboxylic acids, such as, adipic acid,
sebacic acid, phthalic acid, halogenated phthalic acids, maleic acid,
1,2,4-benzenetricarboxylic acid, monomeric, dimeric or trimeric fatty
acids and multifunctional alcohols, such as, for example, ethylene glycol,
polyethylene glycols, propylene glycol, polypropylene glycols, 1,3- and
1,4-butanediol, 2,2-dimethyl-1,3-propanediol, 1,6-hexanediol,
1,1,1-trimethylolpropane, hexanetriols or glycerin.
The reaction of polyisocyanates with the polyether or polyester polyols is
accomplished in a ratio such that, on the average, there are at least 2
isocyanate groups per molecule in the reaction product.
The curing time or the curing temperature of the adhesive can be shortened
or reduced significantly in the conventional manner by the addition of an
accelerator. An especially preferred adhesive is one which contains, as an
accelerator, up to 2 weight percent, based on the sum of the components
(a) and (b) of a tertiary amine and/or an organic compound of tin. As
tertiary amines, dimethylbenzylamine, dicyclohexylmethylamine,
dimethylpiperazine, dimethylaminoethanol, 1,2-dimethylimidazole,
N-methyl-morpholine or N-ethylmorpholine, dimethylcyclohexylamine,
1-aza-bicyclo-(3,3,0)-octane or 1,4-diaza-bicyclo-(2,2,2)-octane are
especially suitable. Examples of readiy usable organic compounds of tin
are dibutyl tin dilaurate or tin(II) octoate.
The inventive adhesive may also contain other auxiliaries. Examples of such
auxiliaries are gluing auxiliaries, such as, for example,
acrylonitrile-butadiene copolymers, polyamides, polychloroprene or
polyurethanes. These gluing auxiliaries are preferably added in an amount
up to 15 weight percent, based on 100 weight percent of components (a) to
(b).
Pigments and/or fillers can also be added to the adhesive as auxiliaries.
For this purpose, inorganic as well as organic products, such as, for
example, titanium dioxide, iron oxide, chromium oxide, barium sulfate,
powdered quartz, talc and bentonite, are suitable.
The following examples illustrate the present invention. They show methods
for the manufacture of the inventive adhesives as well as their adhesion
properties.
Preparation of the inventive adhesives
1. Raw Materials
1.1 Acrylic Copolymers (a)
In order to prepare acrylic copolymers (a) in solution, the monomers
(a.sub.1) to (a.sub.5) (as defined hereinbefore) are reacted in a solvent
mixture of 50 weight percent toluene, 30 weight percent methylethyl ketone
and 20 weight percent of ethyl acetate at 70.degree. to 80.degree. C.,
with the addition of 0.3 to 0.7 weight percent of azobisisobutyronitrile
and 0.4 weight percent of n-dodecyl-mercaptan by the usual solvent
polymerization. The solids content of the copolymer solution is ca. 40
weight percent. The chemical composition of the individual copolymers and
their limiting viscosit,y are given in Table 1.
1.2 Polyisocyanates
Two commercial products are used as polyisocyanates, namely, a
polyphenylpolymethylene-polyisocyanate (crude MDI) of the general formula
##STR3##
hereinafter designated (b1), and a triisocyanate of the formula
##STR4##
hereinafter designated (b2).
Both polyisocyanates are used in the solvent-free form.
Also used is a 60 weight percent solution in methyl ethyl ketone of the
reaction product of 3 moles of 2,4-toluylenediisocyanate and 1 mole of a
trifunctional polyester polyol with a molecular weight of 600, obtained
from adipic acid, ethylene glycol, 2,2-dimethyl-1,3-propanediol and
trimethylolpropane, hereinafter designated (b3).
The free isocyanate content, based on the solids, is:
polyisocyanate (b1): 31.2 weight percent
polyisocyanate (b2): 22.7 weight percent
polyisocyanate (b3): 11.2 weight percent.
Preparation of the Adhesives
For the preparation of the inventive adhesive solutions, the
polyisocyanates are dissolved in the solution of the acrylic copolymers.
Further solvent may be added in order to dilute the adhesive and to adjust
the desired viscosity. Suitable solvents include ketones, such as,
acetone, methyl ethyl ketone, or methyl n-propyl ketone; esters, such as,
ethyl or butyl acetate; or aromatics, such as, toluene or xylene. Because
of the reactivity of water towards isocyanate groups, anhydrous solvents
must be used.
Further additives, such as, accelerators, gluing auxiliaries, pigments and
fillers, may be dissolved or dispersed in the solution obtained.
The exact formulations of the adhesives are given in Table 1. Formulations
1 to 11 describe adhesives in accordance with the present invention. These
are followed by the comparison adhesives, which are numbered 12 to 15.
Formulations 12 and 13 contain the adhesives disclosed in German
Auslegeschrift 25 24 197 or German Pat. No. P 29 26 284.8-43, in which an
acrylic polymer, containing carboxyl groups, is cured with an epoxide
resin. The epoxide resin is based on bisphenol A and epichlorohydrin, and
has an epoxide equivalent of 180 g/gram equivalent.
Instead of the acrylic copolymer, comparison adhesive No. 14 contains a
difunctional polyether polyol, based on propylene oxide and ethylene oxide
and having a hydroxyl number of 90 mg KOH/g.
Instead of the acrylic copolymer, comparison adhesive No. 15 contains a
polyester polyol with a functionality of ca. 2.5 and a hydroxyl number of
109 mg KOH/g. The polyester is obtained by the condensation of adipic acid
with diethylene glycol, ethylene glycol and trimethylolpropane.
The quantities given for the individual components in Table 1 refer to the
solid matter content.
Gluing and Testing the Application of the Adhesives
Two strength tests are used for testing the application of the adhesives:
(a) drum peel strength according to DIN 53 295
(b) angle peel strength according to DIN 53 282.
The drum peel strength is determined on three different composites:
aluminum/polyethylene (PE) composite,
aluminum/epoxide resin and glass fiber laminate (GFK) composite,
aluminum/ABS composite.
Aluminum sheets of grade Al Cu Mg 2pl FH are used. Before being glued, they
are degreased and subjected to a chromate-sulfuric acid pickling. The
polyethylene is roughened and pretreated, and the epoxide resin and glass
fiber laminate (GFK) is ground.
The adhesives are applied in an amount of 60 g/m.sup.2 of solids onto the
adherent, and the solvent is evaporated at room temperature or at an
elevated temperature, for example, at 70.degree. C. The adhesive is cured
for 12 minutes at 90.degree. C. under a pressure of 0.5 N/mm.sup.2. After
cooling, peeling is carried out at 20.degree. C.
The angle strength is determined on samples of roughened and pretreated
polyethylene, 1.2 mm thick. The application of the glue, the evaporation
of the solvents and the curing are carried out as described above. The
angle peel test is carried out at 20.degree. C. In addition, further
samples are stored for three days in hot water at 95.degree. C. Before the
peeling test, these samples are cooled off in water at 20.degree. C. and
peeled while still wet. This storage in hot water is very suitable as a
short-term test for evaluating the resistance of the adhesives to water
and moisture. The strength values obtained are listed in Table 2. It is
evident from the results that the comparison glues No. 12 and No. 13
provide strength values which are considerably less than those of the
inventive glues No. 1 to No. 11. Comparison glues of formulation No. 14
adhere very poorly to plastic surfaces. Admittedly, comparison glues of
formulation No. 15 adhere better to plastic surfaces, however, their
strength decreases greatly after storage in water.
Drum Peel Strength as a Function of the Limiting Viscosity of the Copolymer
(a)
In glue formulation No. 1, acrylic copolymers (a) with different limiting
viscosities are used. The FIGURE shows the drum peel strength of the
aluminum/polyethylene as a function of the limiting viscosity of the
acrylic copolymer (a). It is evident that particular high strengths are
obtained at limiting viscosities ranging from 0.1 to 0.4 (100 ml/g).
TABLE 1
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Glue Formulations
Acrylic Copolymer (a)
Formu- Limiting
Parts
Polyisocyanate
lation
Composition Viscosity
by (b) Molar Ratio
Other Additions
No. Weight % 100 ml/g
Weight
Parts by Weight
OH:NCO Parts by Weight
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1 54.0
ethylacrylate
a.sub.1
0.15 100 18.1 (b1)
1:1.20 --
31.4
acrylonitrile
a.sub.2
14.6
2-hydroxyethyl
a.sub.3
methacrylate
2 30.0
ethylacrylate
a.sub.1
0.18 100 18.9 (b1)
1:1.25 --
24.0
methylmeth-
a.sub.1
acrylate
31.4
acrylonitrile
a.sub.2
14.6
2-hydroxyethyl
a.sub.3
methacrylate
3 54.0
ethylacrylate
a.sub.1
0.18 100 18.0 (b1)
1:1.10 --
32.0
vinylacetate
a.sub.2
14.0
2-hydroxy-
a.sub.3
ethylacrylate
4 41.5
ethylacrylate
a.sub.1
0.27 100 15.5 (b1)
1:1.20 --
10.0
butylacrylate
a.sub.1
36.0
acrylonitrile
a.sub.2
12.5
2-hydroxyethyl
a.sub.3
methacrylate
5 52.4
ethylacrylate
a.sub.1
0.32 100 14.3 (b1)
1:1.20 --
32.5
acrylonitrile
a.sub.2
11.5
2-hydroxyethyl
a.sub.3
methacrylate
3.6
methacrylamide
a.sub.4
6 30.0
ethylacrylate
a.sub.1
0.22 100 19.4 (b1)
1:1.15 --
20.0
methylmeth-
a.sub.1
methacrylate
33.8
acrylonitrile
a.sub.2
16.2
3-hydroxypropyl
a.sub.3
acrylate
7 20.0
methylmeth-
a.sub.1
0.15 100 16.1 (b1)
1:1.10 --
acrylate
15.0
butylmeth-
a.sub.1
acrylate
31.0
acrylonitrile
a.sub.2
14.0
2-hydroxyethyl
a.sub.3
methacrylate
20.0
styrene a.sub.5
8 54.0
ethylacrylate
a.sub.1
0.15 100 24.9 (b2)
1:1.20 0.1 1,4-diazabi-
31.4
acrylonitrile
a.sub.2 cyclo(2,2,2)-
14.6
2-hydroxyethyl
a.sub.3 octane
methacrylate
9 54.0
ethylacrylate
a.sub.1
0.15 100 50.4 (b3)
1:1.20 --
31.4
acrylonitrile
a.sub.2
14.6
2-hydroxyethyl
a.sub.3
methacrylate
10 30.0
ethylacrylate
a.sub.1
0.18 100 18.9 (b1)
1:1.25 20.0 titanium
24.0
methylmeth-
a.sub.1 dioxide
acrylate
31.4
acrylonitrile
a.sub.2
14.6
2-hydroxy-
a.sub.3
ethyl methacrylate
11 30.0
ethylacrylate
a.sub.1
0.18 100 24.9 (b2)
1:1.20 0.1 1,4-diazabi-
24.0
methylmeth-
a.sub.1 cyclo(2,2,2)-
acrylate octane
31.4
acrylonitrile
a.sub.2 10.0 polychloroprene
14.6
2-hydroxyethyl
a.sub.3 moderately strong
methacrylate crystallizing
Mooney plasticity 50
12 60.9
ethylacrylate
0.35 80 -- -- 20.0 epoxide resin
DE-PS
35.4
acrylonitrile
25 24
3.7
acrylic acid
197
13 52.0
ethylacrylate
0.22 71 -- -- 25.4 epoxide resin
P 29 26
33.0
acrylonitrile 3.6 pyromellitic
284.8-
15.0
acrylic acid dianhydride
43
14 polyether polyol
-- 100 25.9 (b1)
1:1.20 --
15 polyester polyol
-- 100 31.4 (b1)
1:1.20 --
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TABLE 2
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Testing the Application of the Glues
Angle Peel Strength
DIN 53 282 N/mm
After
Storing
for 3
Glue Drum Peel Strength DIN Days in
Formulation
53 295 Nmm/mm Without Hot
No. Al/Pe Al/GFK Al/ABS Stress Water
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1 315 65 85 4.8 4.5
2 305 76 110 5.3 5.2
3 310 71 90 4.5 4.0
4 285 50 80 4.9 4.8
5 290 48 88 5.4 5.0
6 285 53 105 5.0 4.7
7 315 65 88 4.8 4.6
8 270 60 80 4.7 4.5
9 265 64 88 4.8 4.6
10 295 56 106 5.1 5.0
11 290 54 98 4.6 4.5
Comparison
Glue
12 45 20 27 1.2 1.0
13 70 28 39 1.4 1.2
14 5 3 5 0.1 0.07
15 145 39 48 3.0 0.73
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Description |
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