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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to organoborane amine complexes and, more
particularly, to polymerizable compositions, especially acrylic adhesives,
that incorporate polymerization initiator systems based on the
organoborane amine complexes. This invention further relates to methods
for bonding substrates, particularly low surface energy substrates, using
such compositions.
2. Description of the Related Art
An efficient, effective means for adhesively bonding low surface energy
substrates such as polyethylene, polypropylene and polytetrafluoroethylene
(e.g., TEFLON) has long been sought. The difficulties in adhesively
bonding these materials are well known. See, for example, "Adhesion
Problems at Polymer Surfaces" by D. M. Brewis that appeared in Progress in
Rubber and Plastic Technology, volume 1, page 1 (1985). The conventional
approaches typically function by: (1) increasing the surface energy of the
substrate (to more closely match the surface energies of the substrate and
the adhesive thereby promoting better wetting of the substrate by the
adhesive) and/or (2) eliminating additives and low molecular weight
polymer fractions in the substrate that can migrate to the substrate
surface and adversely affect adhesion by forming a weak boundary layer.
As a result, the conventional approaches often use complex and costly
substrate surface preparation techniques such as flame treatment, corona
discharge, plasma treatment, oxidation by ozone or oxidizing acids, and
sputter etching. Alternatively, the substrate surface may be primed by
coating it with a high surface energy material. However, to achieve
adequate adhesion of the primer, it may be necessary to first use the
surface preparation techniques described above. All of these techniques
are well known, as reported in Treatise on Adhesion and Adhesives (J. D.
Minford, editor, Marcel Dekker, 1991, New York, volume 7, pages 333 to
435). The known approaches are frequently customized for use with specific
substrates. As a result, they may not be useful for bonding low surface
energy substrates generally.
Moreover, the complexity and cost of the presently known approaches do not
render them particularly suitable for use by the retail consumer (e.g.,
home repairs, do-it-yourselfers, etc.) or in low volume operations. One
vexing problem is the repair of many inexpensive everyday household
articles that are made of polyethylene, polypropylene or polystyrene such
as trash baskets, laundry baskets and toys.
Consequently, there has been a considerable and long felt need for a
simple, easy to use adhesive that can readily bond a wide variety of
substrates, especially low surface energy materials, such as polyethylene,
polypropylene and polytetrafluoroethylene, without requiring complicated
surface preparation, priming and the like.
This invention is directed to polymerization initiator systems based on
organoborane amine complexes and adhesives and other compositions made
therewith. The adhesives are especially useful in bonding low surface
energy substrates such as polyethylene, polypropylene and
polytetrafluoroethylene.
In 1957 G. S. Kolesnikov et al. (Bull. Acad. Sci. USSR, Div. Chem. Sci.
1957, p. 653) reported the use of tributylborane as a catalyst for the
polymerization of styrene and methyl methacrylate. The addition of 2 mole
% of tributylborane to methyl methacrylate resulted in rapid
polymerization; a transparent solid block was formed in 60 to 90 minutes.
At about the same time, J. Furakawa et al. (Journal of Polymer Science,
volume 26, issue 113, p. 234, 1957) reported that triethylborane had been
found to initiate the polymerization of some vinyl compounds such as vinyl
acetate, vinyl chloride, vinylidene chloride, methacrylic ester, acrylic
ester, and acrylonitrile. J. Furakawa et al. (Journal of Polymer Science,
volume 28, issue 116, 1958) later reported that triethyl borane-catalyzed
vinyl polymerization could be markedly accelerated with oxygen or oxygen
compounds such as hydrogen peroxide and metal oxides. While the presence
of oxygen is apparently needed for the polymerization to occur, the
organoborane compounds of the type described in these references are known
to be quite pyrophoric in air. Hence, the presence of oxygen is
simultaneously required and undesirable.
U.S. Pat. No. 3,275,611 "Process for Polymerizing Unsaturated Monomers with
a Catalyst Comprising an Organoboron Compound, a Peroxygen Compound and an
Amine" issued Sep. 27, 1966 to E. H. Mottus et al. discloses a process for
polymerizing olefinic compounds, especially alpha-olefinically unsaturated
compounds. Particularly preferred are methacrylate monomers having no more
than 20 carbon atoms in the ester group. The organoboron compound and the
amine may be added to the reaction mixture separately or they may be added
as a preformed complex. The latter approach reportedly has the advantage
of making the boron compound more easily handled, especially for certain
boron compounds that tend to be pyrophoric in air but which are not
pyrophoric when complexed. Especially useful boron catalysts are said to
have the following general formulas: R.sub.3 B, RB(OR).sub.2, R.sub.2
B(OR), R.sub.2 BOBR.sub.2, R.sub.2 BX, and R.sub.2 BH, where R is a
hydrocarbon radical, preferably an alkyl radical having from 1 to 10 or
more carbon atoms (more preferably, up to 6 carbon atoms), and X is a
halogen.
Useful amine complexing agents are said to have a basicity that is
preferably in the range of about 10.sup.-6 or 10.sup.-7 to
5.times.10.sup.-10 or 10.sup.-10. Various amine complexing agents are
mentioned although pyridine, aniline, toluidine, dimethylbenzylamine, and
nicotine are used in the examples. The amine and boron compounds are used
in about a 1:1 molar ratio, assuming one nitrogen function per boron
function. Reportedly, any peroxide or hydroperoxide compound may be used
as a catalyst component.
While Mottus et al. refer to polymerizing methacrylate monomers, there is
no indication that the resulting polymers are useful as adhesives. Various
acids are mentioned as monomers that may be polymerized but there is no
indication that an acid is a component of the polymerization system.
British Patent Specification No. 1,113,722 "Aerobically Polymerisable
Compositions," published May 15, 1968 discloses the polymerization of
acrylate monomers through the use of a free-radical catalyst (e.g.,
peroxides) and triarylborane complexes having the general formula
(R.sub.3)B-Am wherein R is an aryl radical having from 6 to 12 carbon
atoms and Am is, among other things, an amine such as hexamethylenediamine
or ethanolamine. The polymerization is activated by heating or the
addition of an acid. The resulting compositions are reportedly useful as
adhesives.
Chemical Abstracts No. 88532r (volume 73, 1970) "Dental Self-curing Resin"
and the full text paper to which it refers report that tributylborane can
be made stable in air by complexing it with ammonia or certain amines
(e.g., aniline, n-butylamine, piperidine, ethylenediamine) at a mole ratio
of one and that the tributylborane can be reactivated with an amine
acceptor such as an isocyanate, an acid chloride, a sulfonyl chloride, or
anhydrous acetic acid. As a result, the complex can be used to polymerize
blends of methyl methacrylate and poly(methylmethacrylate) to provide a
dental adhesive. Tributylborane-ethylenediamine complexes and
triethylborane-ammonia complexes, each with p-toluenesulfonyl chloride as
the amine acceptor, are specifically mentioned.
Chemical Abstracts No. 134385q (volume 80, 1974) "Bonding Polyolefin or
Vinyl Polymers" reports that a mixture of 10 parts methyl methacrylate,
0.2 parts tributylborane, and 10 parts poly(methylmethacrylate) was used
to bond polyethylene, polypropylene and poly(vinyl acetate) rods.
U.S. Pat. No. 5,106,928 "Acrylic Adhesive Composition and Organoboron
Initiator System," issued Apr. 21, 1992 to M. M. Skoultchi et al.,
discloses a two-part initiator system that is reportedly useful in acrylic
adhesive compositions, especially elastomeric acrylic adhesives. The first
part of the two-part initiating system is a stabilizing organoborane amine
complex; the second part is an organic acid activator. The organoborane
compound of the complex has the general formula:
##STR2##
where R, R.sub.1 and R.sub.2 are either alkyl groups having 1 to 10 carbon
atoms or phenyl groups, although alkyl groups of 1 to 4 carbon atoms are
preferred. The amine portion of the complex may be ammonia, a primary
amine, a secondary amine, or a polyamine containing a primary amine or a
secondary amine. Useful amines include n-octylamine, 1,6-diaminohexane,
diethylamine, dibutylamine, diethylenetriamine, dipropylenediamine,
1,3-propylenediamine, and 1,2-propylenediamine.
The organic acid activator is a compound that will destabilize or liberate
the free organoborane compound by removing the amine group, thereby
allowing it to initiate the polymerization process. Preferably, the
organic acid has the formula R--COOH where R is hydrogen, an alkyl or
alkenyl group having 1 to 8 (preferably 1 to 4) carbon atoms, or an aryl
group with 6 to 10 (preferably 6 to 8) carbon atoms.
Twelve organoborane amine initiator complexes are described in conjunction
with Example I. In those complexes based on diamines or triamines, the
nitrogen atom to boron atom ratio ranges from 2:1 to 4:1. In those
complexes based on diethylamine and n-octylamine, the nitrogen atom to
boron atom ratio is 1.5:1
The adhesive compositions are reportedly particularly useful in structural
and semi-structural applications such as speaker magnets, metal-metal
bonding, (automotive) glass-metal bonding, glass-glass bonding, circuit
board component bonding, selected plastic to metal, glass, wood, etc. and
electric motor magnets. Those plastics that may be bonded are not further
described.
SUMMARY OF THE INVENTION
The invention relates to polymerizable acrylic compositions, especially
acrylic adhesives, that incorporate polymerization initiator systems based
on organoborane amine complexs. The adhesives are particularly useful in
bonding low surface energy substrates (e.g., polyethylene, polypropylene,
polytetrafluoroethylene, etc.) that, heretofore, have been bonded using
complex and costly surface preparation techniques.
The polymerizable acrylic compositions comprise and, more preferably,
consist essentially of at least one acrylic monomer (preferably alkyl
acrylates such as butylacrylate and/or alkyl methacrylates such as
methylmethacrylate), an effective amount of an organoborane amine complex,
and an effective amount of an organic or inorganic acid (e.g., acrylic
acid, methacrylic acid or SnCl.sub.4) for initiating polymerization of the
acrylic monomer.
Useful organoborane amine complexes have the following general formula:
##STR3##
wherein: R.sup.1 is an alkyl group having 1 to 10 (preferably 2 to 5)
carbon atoms;
R.sup.2 and R.sup.3 are independently selected from phenyl-containing
groups and alkyl groups having 1 to 10 carbon atoms, alkyl groups having 2
to 5 carbon atoms being preferred;
R.sup.4 is selected from the group consisting of CH.sub.2 CH.sub.2 OH and
(CH.sub.2).sub.x NH.sub.2 wherein x is an integer greater than 2,
preferably from 2 to 6, and most preferably 6;
R.sup.5 is hydrogen (preferred) or an alkyl group having 1 to 10 carbon
atoms; and
the nitrogen atom to boron atom ratio is about 1:1 to 2:1, more preferably
about 1:1 to 1.5:1, and most preferably about 1:1.
The organoborane amine complex is typically provided in an amount of about
0.15 to 3 mole % based on the number of moles of acrylic groups, moieties
or functionality (more preferably about 0.2 to 2.5 mole %; most preferably
about 1 to 1.5 mole %). An effective amount of the acid is about 30 to 540
mole % (most preferably about 230 mole %), based on the number of
equivalents of amine groups, moieties or functionality.
Among the useful additives that may be optionally included within these
compositions are thickening agents (such as polymethylmethacrylate) and a
small amount (about 0.1 to 7 mole % based on the number of moles of
acrylic functionality) of a substantially uncomplexed organoborane, the
latter being especially useful if the organoborane amine complex is based
on monoethanolamine.
In another aspect the invention relates to a method for bonding low surface
energy polymeric substrates using the polymerizable acrylic compositions
described above. The substrate surface may first be primed with a
composition comprising the organoborane amine complex in an inert organic
solvent (e.g., to about 5 to 15 wt. %), in which case the inclusion of the
organoborane amine complex in the polymerizable composition is optional.
In yet another aspect of the invention, certain compositions are useful as
primers for increasing the adhesion of a subsequently applied adhesive to
fluoroplastic substrates. Among such useful primers are those based on
acrylic monomers, organoboranes and an oxygen source (e.g. peroxides or
atmospheric oxygen) as well as those based on acrylic monomers,
organoborane amine complexes, and acids.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In a broad aspect, this invention relates to polymerizable acrylic
compositions, especially acrylic adhesives, that are produced using
polymerization initiator systems based on organoborane amine complexes.
The adhesives are particularly useful in bonding low surface energy
substrates (e.g., polyethylene, polypropylene, polytetrafluoroethylene,
etc.) that, heretofore, have been bonded using complex and costly surface
preparation techniques.
The polymerization initiator systems useful in the invention comprise and,
more preferably, consist essentially of an effective amount of an
organoborane amine complex and an effective amount of an acid for
liberating the organoborane to initiate polymerization. Organoborane amine
complexes useful in the invention have the following general structure:
##STR4##
where R.sup.1 is an alkyl group having 1 to 10 carbon atoms and R.sup.2
and R.sup.3 are independently selected from alkyl groups having 1 to 10
carbon atoms and phenyl-containing groups. More preferably, R.sup.1,
R.sup.2 and R.sup.3 are alkyl groups having 1 to 5 carbon atoms such as
methyl, ethyl, propyl, iso-propyl, n-butyl, iso-butyl, and pentyl. In
general, shorter carbon chain lengths are preferred for the R.sup.1,
R.sup.2 and R.sup.3 groups as this promotes enhanced stability of the
complex in air. Smaller, less bulky substituents are also preferred as
larger, more bulky groups may negatively affect adhesion. By
"independently selected" it is meant that R.sup.2 and R.sup.3 may be the
same or that they may be different. R.sup.1 may be the same as R.sup.2 or
R.sup.3, or it may be different. Preferably R.sup.1, R.sup.2 and R.sup.3
are the same. The tripropyl, tri-iso-propyl, and tri-n-butyl alkylboranes
have been found to be especially useful.
The amine component of the complex may be either monoethanolamine, a
primary alkyl diamine, or a secondary alkyl diamine. Consequently, R.sup.4
may be selected from the group consisting of CH.sub.2 CH.sub.2 OH and
(CH.sub.2).sub.x NH.sub.2, wherein x is an integer greater than 2. R.sup.5
is either hydrogen or an alkyl group having 1 to 10 carbon atoms. In more
preferred complexes, R.sup.5 is hydrogen (so as to reduce the steric
hinderance within the organoborane amine complex which could inhibit the
formation of the complex itself) and R.sup.4 is either CH.sub.2 CH.sub.2
OH or (CH.sub.2).sub.x NH.sub.2 where x is an integer from 2 to 6. Most
preferred, however, are complexes where the R.sup.4 is CH.sub.2 CH.sub.2
OH (monoethanolamine) or (CH.sub.2).sub.6 NH.sub.2
(1,6-hexamethylenediamine).
Importantly, and as shown more fully hereinbelow, the nitrogen atom to
boron atom ratio in the complex is about 1:1 to 2:1, more preferably about
1:1 to less than 2:1, even more preferably about 1:1 to 1.5:1, and most
preferably about 1:1. At nitrogen atom to boron atom ratios in excess of
2:1 the practical utility of the complex in a polymerization initiator
system is diminished as the amount of complex that must be employed to
achieve a useful molecular weight during polymerization becomes too large.
On the other hand, a nitrogen atom to boron atom ratio of less than 1:1
leaves free organoborane, a material that tends to be pyrophoric.
An effective amount of the organoborane amine complex is an amount that is
large enough to permit polymerization to readily occur to obtain an
acrylic polymer of high enough molecular weight for the desired end use.
If the amount of organoborane amine complex is too high, then the
polymerization may proceed too rapidly to allow for effective mixing and
application of the composition. The useful rate of polymerization will
depend in part on the method of applying the composition to the substrate.
Thus, the rate of polymerization for a high speed automated industrial
applicator can be faster than if the composition is applied with a hand
applicator or if the composition is mixed manually.
Within these parameters, an effective amount of the organoborane amine
complex is about 0.15 to 3 mole %, based on the number of moles of acrylic
functionality, more preferably about 0.2 to 2.5 mole %, and most
preferably about 1 to 1.5 mole %. If the amine is provided by
monoethanolamine, it has been found that an effective amount of the
complex is greater than 2 mole % but less than about 5 mole %. By "acrylic
functionality" is meant acrylic and substituted acrylic moieties or
chemical groups; that is, groups which have the general structure
##STR5##
wherein R and R' are organic radicals that may be the same or that may be
different.
As explained below, however, it is sometimes advantageous to further
include a small amount of additional, substantially uncomplexed
organoborane. In these instances, an effective amount of the complex is
about 0.3 to 5 mole %, based on the number of moles of acrylic
functionality, more preferably about 0.5 to 4 mole %, and most preferably
about 1 to 3 mole %.
Advantageously, the organoborane amine complexes useful in the invention
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