Note: Descriptions are shown in the official language in which they were submitted.
-'- 2i 99516
DUAL-CURE LATEX COMPOSITIONS
BACKGROUND OF THE 1NVENT10N
The present invention relates generally to latex compositions which are cured
by exposure to actinic radiation. Such latex compositions are especially
useful in
wood and wood product coatings applications, as binders for inks and overprint
varnishes, and as adhesives. The present invention relates particularly to
such
radiation-curable compositions having a secondary curing mechanism which is
not
dependent upon exposure to radiation.
The primary advantages to radiation-curable compositions are: speed of
curing; stability; and the process control afforded to the user, especially in
high
speed, automated processes. These advantages are-offset, however, by some
significant disadvantages, most notably the inability of ultraviolet (UV)
radiation to
penetrate through the composition itself, and the inability to cure in
unexposed or
"shadow' regions. In either circumstance, the end result is a coating which is
uncured, or undercured.
Others have attempted to overcome these disadvantages by, infer alia,
providing secondary curing mechanisms which are not dependent upon exposure to
actinic radiation. Such products are generally referred to as "dual cure'
products.
Examples of such secondary mechanisms include: heat-curing, using thermal
initiators such as peroxides, azo compounds, and disulfides; anaerobic curing,
wherein radical initiators (such as peroxides) which initiate slow
polymerization
reactions on exclusion of air; aerobic curing, using metal driers to initiate
oxidative
curing; and moisture-curing, using isocyanates or oxazolidines which react
with
ambient moisture to effect curing. These secondary curing mechanisms are
reviewed by John G. Woods in Chapter 9 ("Radiation Curable Adhesives") of
Radiation Curing: Science and Technology Plenum Press:New York,1992, pp. 333-
398.
Reactions of epoxy groups with various non-epoxy functional groups,
including carboxylic acids, have been used toattach pendant unsaturation to
polymer chains to render them curable with actinic radiation; however, in such
cases, high manufacturing temperatures are generally available to accelerate
the
epoxy reaction rate. The epoxy-acid reaction is very slow at ambient
temperatures,
and consequently, is not considered to be suitable for use as a secondary
curing
mechanism for most radiation-curable coatings and adhesives, since the heat
sensitivity of the substrates employed prevents them from being cured at high
temperatures. See, for example: Ullmanri s Encyclopedia of Industrial
Chemistry
_2_
2J99575
Fifth ed., vol. A9, p. 556; see also data in G. Walz paper, in Proceedings of
XIth
International conference in Or~ariic Coatings Science and Technology (8-12
July
1985), Athens, Greece; p. 429ff.
STATEMENT OF THE LN VENTION
One aspect of the present invention is directed to radiation-curable latex
compositions having a secondary curing mechanism, comprising: an anionically
stabilized, water-borne dispersion of one or more radiation-curable resins;
and a
low molecular weight compound having at least two reactive functional groups,
wherein one reactive functional group comprises an epoxy and the other
reactive
functional group comprises either an epoxy or a functionality capable of self
condensation after film formation.
Another aspect of the present invention is directed to a method for providing
a secondary curing mechanism to a radiation-curable latex composition,
comprising
the addition of a low molecular weight compound having at least two reactive
functional groups, wherein one reactive functional group comprises an epoxy
and
the other reactive functional group comprises either an epoxy or a
functionality
capable of self-condensation after film formation.
A third aspect of the present invention includes a method for providing a
cross-linked protective coating on a substrate, comprising the steps of:
applying a
coating of the composition of the present invention to the substrate; exposing
the
coated substrate to actinic radiation to effect curing; and allowing the
unexposed or
underexposed portions of the coated substrate to cure at room temperature or
grea ter.
DETAILED DESCRn'TION OF THE INVENTION
As used in this specification, the following terms have the following
definitions, unless the context clearly indicates otherwise. "Late~C' or
"latex
composition° refers to a dispersion of a water-insoluble polymer which
may be
prepared by conventional polymerization techniques such as, for example, by
emulsion polymerization, and "resin' refers to the polymer in the latex.
"Crosslinkable' and "crosslinking' refer to the formation of new chemical
bonds
between existing polymer chains, and "curing" refers to the crosslinking of
polymers after application to the substrate. "Storage-stable' refers to the
ability of a
latex composition or formulation to maintain its physical state and
application
characteristic, and give films with reproducible properties, during periods of
prolonged storage in a storage container, prior to application to a substrate.
"Pot
life' or "shelf life' refers to the period of time a composition is storage-
stable.
2199576
"Two-pack" or "two-component' refers to coating compositions (or systems) with
a
relatively short pot life: In genei=al, the components of two-component
systems are
stored separately, then are mixed together shortly before use. On the other
hand,
"one-pack" or "one-component' refers to coating compositions with a long shelf
life,
such that the components may be stored together in one container. Ranges
specified
are to be read as inclusive, unless specifically identified otherwise.
hi the present invention, the resins of the present invention include but are
not limited to: addition polymers of at least one ethylenically unsaturated
monomer; condensation polymers made by the reaction of one or more
diisocyanates or polyisocyanates with one or more compounds containing groups
with active hydrogens; and polyester resins made by the reaction of one or
more
alcohols, especially diols or polyols, with polyhydric acids or anhydrides of
polybasic acids. Such addition polymers include, for example, those prepared
from
acrylic ester monomers-including methyl acrylate, ethyl acrylate, butyl
acrylate, 2-
ethylhexvl acrylate, methyl methacrylate, butyl methacrylate; styrene or
substituted
styrenes; butadiene; vinyl acetate or other vinyl esters; vinyl monomers such
as
chloride, vinylidene chloride, N-vinyl pyrrolidone; and acrylonitrile or
methacrylonitrile. The condensation polymers include, for example,
polyurethanes
and polyureas such as those made by the reaction of one or more diisocyanates
or
polyisocyanates with one or more compounds containing groups with active
hydrogens such as, for example, polyester, polycarbonate, or polyether di or
polyols, monomeric alcohols, diols or polyols, primary or secondary amines or
hydrazine compounds, mercaptans, or compounds containing enolic hydrogens
such as acetoacetate groups; likewise included are polyester resins made by
the
reaction of one or more alcohols, especially diol~ or polyols, with polyhydric
acids
or anhydrides of polybasic acids, such as, for instance, reaction products of
ethylene
glycol, propylene glycol, the isomeric butanediols or hexanediols, glycerol,
neopentylglycol, allyl alcohol, trimethylolpropane, diethylene glycol,
triethylene
glycol, dipropylene glycol, or polyether oligomers made by the condensation of
one
or more of these alcohols, with acids or acid anhydrides such as adipic acid,
malefic
acid, malefic anhydride, phthalic acid, phthalic anhydride, tetrahydrophthalic
acid,
tetrahydrophthalic anhydride, trimellitic anhydride, acrylic acid, methacrylic
acid,
fumaric acid, itaconic acid, or natural oil fatty acids such as linseed oil
fatty acids,
tall oil fatty acids, soybean oil fatty acids, or abietic acid. Polyester
resins or their
precursors may also be made using transesterification-reactions using methods
well
known in the art for the production of alkyd polyesters.
Dispersions of these resins may be in the form of single or mufti-staged
particles. Mufti-staged particles will comprise at least two mutually
incompatible
CA 02199576 2005-O1-04
copolymers having any of a number of morphological configurations - for
example: core/shell; coref shell particles with shell stages incompletely
encapsulating the core; core/shell particles with a multiplicity of cores,
interpenetrating network particles; and the like, where the greater portion of
the
surface area of the particles will be occupied by at least one outer stage,
and the
interior of the particle will be occupied by at least one inner stage.
For addition polymers included in. the present invention, anionic
stabilization
may be conferred through the copolymerization of low levels of ethylenically
unsaturated acid monomers (e.g.; 0.1-7~0, by weight, based on the weight of
the
addition polymer). Examples of ethylerucally unsaturated acid monomers useful
in
the.present invention include but are not limited to those of: acrylic acid,
methacrylic acid, crotonic acid, itaconic acid, fumaric acid, malefic acid,
monomethyl
itaconate; nionomethyl fumarate, malefic anhydride, 2-acrylamido-2-methyl-1-
propanesulforuc acid, sodium vinyl sulfonate, and phosphoethyl methacrylate.
For polyurethane condensation polymers included in the present invention,
anionic stabilization may be conferred through the copolymerization of acid-
containing compounds into the polymer backbone, such as, for example, 0.1 -15
wt~, based on the weight of the polyurethane polymer, of dimethylolpropionic
acid
or of its sulfonic acid analogue. For polyester condensation polymers included
in
the present invention, anionic stabilization may be conferred through the, use
of a
molar excess of acid functional groups during the polymerization of the.resin,
such
that the resin has an acid equivalent weight between about 600 and 20,000 (for
water-reducible resins, preferably between about 900 and 1400).
The polymers are rendered radiation-curable through the incorporation of
eEhylenically unsaturated groups, which may either be directly incorporated
into the
polymer backbone during its manufacture, or attached to the polymer backbone
at
some subsequent point. Examples of aruonically stabilized, radiation-curable
polymers useful in the present invention include but are not limited to those
disclosed and described in: US 4,287,039 (Buethe, et al.); DE 4,011,353 and DE
4,011,349(Kressdorf et al.), DE 4,031,732 and DE 4,203,546 (Beck et al.); EP
399,160
(Flakus), E1' 392,352 (Haberle et al.), EP 518,020 (Flakus); US 5,306,744
(Wolfersberger et al.) ,US 4,730,021 (Zom et al.), US 4,107,013 (McGinruss, et
al.), US
5,371,148 (T~ylor et al.), WO 95 j00560 (Johnson et al.), and EP 442,653
(Pears, et al.).
Depending on the particular use, the resins useful in the present invention
will general ly be supplied as aqueous dispersions at solids levels between
about 20
2799576
wt% and 70 wt%, or in water-reducible form (with or without a cosolvent) at
solids
levels between about 50 wt% and 100 wt%: The level of solids preferred for
coatings
applications depends upon the requirements of the particular application. For
those
applicarions where a low solids coating is preferred, it is preferred to use
formulations between 5 wt% and 60 wt% of polymer solids, most preferably
betGVeen about 20 wt% and 50 wt% . High solids coarings are preferably
formulated
at solids levels in excess of 60%, most preferably between 80 and 100 wt%.
The low molecular weight, epoxy-containing compounds of-the present
invention contaiaeither: at least two epoxy functional groups (i.e. groups
containing an oxicane ring); or at least one epoxy group and at least one
other
functional group capable of undergoing a condensation reaction with itself or
with
some reactive functionality on the resin backbone. The molecular weight of
such
compounds is preferably less than 1000, most preferably in the range of 100 -
500.
Preferred epoxy-containing compounds include but are not limited to: aliphatic
or
cycloallphatic di- and tri-epoxies such as 3,4-epoxycyclohyexylmethyl-3,4-
epoxycyclohexane carboxylate or bis-(3,4-epoxycyclohexyl) adipate; and
epoxysilanes such as 3-glycidoxypropyltrimethyoxysilane or other
glycidoxyalkyl
trialkoxysilanes.
The epoxy compounds are added to the resin using methods known to those
skilled in the art. For one-pack compositions, the simplest method is to add
slowly
an appropriate amount of the epoxy compound to the appropriate amount of resin
under conditions of good distributive mixing, then to continue stirring for a
period
of time, typically 10 minutes to three hours. For two pack compositions, the
epoxy
compound may be added by the end user, under conditions of good distributive
mixing; to a previously formulated paint, varnish or coating. In such cases,
it may
be preferable to let the epoxy-resin blend equilibrate several hours or
overnight
before application to the substrate. Potlifes obtainable with the compositions
of the
present invention may be several weeks.
Two pack compositions may also be mixed using plural component
application equitiment, in-line mixers, and so forth, using mixing and
application
methods which are well known in the art.
Typical use levels for epoxy compounds of the present invention are between
0.2 -1.5 epoxy equivalents per resin acid equivalent, preferably between 0.5 -
1.0
epoxy equivalents per resin acid equivalent, depending on the epoxy, and the
particular use for the resultant latex. The resin acid equivalent weights may
be
determined by a direct titration method such as that described in ASTM D4370-
84,
or alternatively, acid numbers supplied by manufacturers may be used. On a
2? 9956
weight basis, epoxy compound levels may work out to be between about 0.5 and
10
wt%, based on the total weight of the polymer.
Surfactants are commonly used in emulsion or dispersion polymerization to
provide stability, as well as to control particle size. Surfactants can also
provide
dispersibility for water-reducible resins. Conventional surfactants include
anionic
or nonionic emulsifiers or combinations thereof. Typical anionic emulsifiers
include
but are not limited to: alkali or ammonium alkyl sulfates, alkyl sulfonates,
salts of
fatty acids, esters of sulfosuccinic acid salts, alkyl diphenylether
disulfonates, and
salts or free acids of complex organic phosphate esters. Typical nonionic
emulsifiers
include but are not limited to: polyethers, e.g. ethylene oxide and propylene
oxide
condensates which include straight and branched chain alkyl and alkylaryl
polyethylene glycol and polypropylene glycol ethers and thioethers, alkyl
phenoxypoly(etlayleneoxy) ethanols having alkyl groups containing from about 7
to
about 18 carbon atoms and having from about 4 to about 100 ethyleneoxy units,
and
polyoxy-alkylene derivatives of hexitol, including sorbitans, sorbides,
mannitans,
and mannides. Surfactants may be employed in the compositions of the present
invention at levels of 0.1 - 3 wt% or greater, based on the total weight of
the final
composition.
Compositions of the present invention may contain photoiniriators, or
combinations of photoinitiators and photoactivators, to promote the curing of
the
coating in those areas of the coating which are exposed to actinic radiation.
Typical
use levels for photoinitiators are 0.1 - 6 wt% based on non-volatile material,
preferably about 0.5 -4.0 wt%a. Examples of such photoinitiators include
benzophenone and substituted benzophenones, benzoin and its derivatives such
as
benzoih butyl ether and benzoin ethyl ether, benzil ketals such as ben2il
dimethyl
ketal, acetophenone derivatives such as a,a-diethoxyacetophenone and a,a-
dimethyl-a-hydroxyacetophenone, benzoates such as methyl-o-benzoyl benzoate,
thioxanthones, lvlichler's ketone, and acylphosphine oxides or bis-
acylphosphine
oxides.
Other optimal components of the compositions of the present inverition include
but are not limited to: co-solvents and coalescents, pigments, fillers,
dispersants,
wetting agents, <::vH-foam agents, UV absorbers, antioxidants, biocides, and
stabilizers.
These optional components (as desired) may be added in any order of addition
which
does not cause an incompatibility between components. Components which do not
dissolve in the aqueous carrier (such as pigments and fillers) can be
dispersed in the
latex or an aqueous carrier or co-solvent using a high shear mixer. The pH of
the
composition can be adjusted by adding acid or base, with agitation. Examples
of base
include but are not limited to ammonia, diethylamine, friethylamine,
2~9951b
dimethylethanolamine, triethanolamine, sodium hydroxide, potassium hydroxide,
and
sodium acetate. Examples of acids include but are not limited to acetic acid,
formic
acid, hydrochloric acid, nitric acid, and toluene sulfonic acid.
The formulated coating compositions may be used as top coats, intermediate
coats, or primer coats, and are useful as: paints, including wood lacquers;
stains;
varnishes; adhesives; inks, including screen printing inks and gravure and
flexographic printing inks; plastics, including 'plastic sheeting and
polyvinylchloride
flooring; fiber; paper, including overprint varnishes for paper and board;
leather;
and solder mask photoresists on electronic circuits, printing plates, and
other
composites using ultraviolet curing. These coatings are particularly useful in
wood
applications, such as for example, on cabinets, furniture, and flooring.
The compositions of the present invention can be applied to desired
substrates using conventional application techniques such as conventional or
airless
spray, roll, brush, curtain, flood, bell, disc, and dip-coming methods. Once
applied
to the substrate, the compositions are cured by exposure to radiation after
most or
all of the water has evaporated from the composition. Useful forms of
radiation
include ionizing radiation, electron beam radiation, and ultraviolet
radiarion.
Sources of ultraviolet radiation include sunlight, mercury vapor lamps, carbon-
arc
lamps, xenon lamps, and the like. It is preferred to use mercury vapor lamps.
The following examples are presented to illustrate further various aspects of
the present invention, but are not intended to limit the scope of the
invention in any
respect. In Examples 1 and 2, below, mar resistance and spot resistance tests
were
conducted on various latexes, with and without the secondary curing mechanisms
of
the present invention, for comparison purposes. The test methods and the
formulations for the latex controls used in these Examples are described
below.
Mar Resistance Test - The film-is struck vigorously with the back of the
fingernail several times, then rated according to the mark left on the film.
Spots
were rated visually on a 0-10 scale, where 10 indicates no trace left on the
film.
Spot Resistance Test - Covered spot tests were performed according to
ASTMD130$-87 . Spots were rated visually after recovery using a 0-10 scale,
where
0 = complete destruction of the coating, and 10 = no effect of test solution.
Latex A is a radiation-curable acrylic latex, formed by making a two stage
polymer of overall composition 48 wt% butyl acrylate, 24 wf% styrene, 25.5 wt%
methacrylic acid, and 2.5% allyl methacrylate, neutralizing 15% of the acid
equivalents with ammonium hydroxide, adding an amount of glycidyl methacrylate
corresponding to 74 mole percent of the acid, and reacting at about 80
°C until
essentially all the glycidyl methacrylate has reacted. The resulting latex had
a solids
2? 99576
_8_
content of 40.2% by weight, a methacrylate equivalent weight of 592 based on
dry
polymer (for UV curing), and an acid number of 58 based on dry polymer.
Latex t~ for purposes of comparison, is a non-radiation-curable latex, formed
by preparing a single stage polymer of butyl acrylate and methyl methacrylate,
with
a glass transition temperature of 55 °C. The. resulting latex had a
solids content of
37% by weigh, no residual methacrylate functionality, and an acid number of 52
based on dr~~ polymer. It was neutralized with ammonia to pH 7.0 and
formulated
according to the following table.
' INGREDIENT Amount (wt%)
Latex B 202.7
Ethylene glycol monobutyl 9.75
ether
Ethylene glycol ethylhexyl 1.50
ether
Isopropanol 11.25
Water 63.1
7% Ammonia solution 20.0
Example 1: Latex with Epoxysilane
For this example, 3-glycidyoxypropyltrimethoxysilane ("GPMS") was added
to Latex A, at levels of 25% and 50% equivalents (2.3 wE% and 4.5 wt%
respectively,
based on the weight of the wet Latex A). Solids were kept constant at 40% by
addition of water, as necessary. The epoxysilane stirred in readily, without
any
apparent shock to the latex. The adduct preparations remained fluid, without
sludge-or apparent viscosity buildup, for at least several weeks. When the
formulations were 7 days old, they were applied to cherry veneer using a draw
down bar (two coats). Latex A alone (without photoinitiator) and Latex B not a
radiation-curable thermoplastic formulation) were-also applied, as controls.
The
films were then aged 3 days at 60 °C in order to simulate an extended
room
temperature cure. The films were then tested, and gave the following results.
2199~7b
-9-
Latex A Latex A + 0.25eqLatex A +
-
Alone GPMS OSeq GPMS Latex
B
16 )your spot test:
Water 9 9 10 10
1 % Dreft detergent5 8 8 7
Vinegar 9 10 10 9
1 lour spot test:
50% EtOH 8 ~ 10 4
3A EtOH 1 7 8 0
7%a Ammonia 1 3 4 1
Mar Resistance 6 6 J 4
'
While the Latex A alone performed well after the three day accelerated cure,
the boost from the epoxysilane was quite evident. With the addition of the
epoxysilane, the Latex A films surpassed the Latex B film by a wide margin in
base
resistance, alcohol resistance, and mar resistance.
Example 2: Latex with Diepoxy
For this example, an aliphatic diepoxy (3,4-epoxycyclohexylmethyl-3,4-
cyclohexylcarboxylate) was stirred into Latex A. Formulations at 50% and 100%
equivalents (2.7 wt%a and 5.3 wt%a respectively, based on the weight of the
wet Latex
A) were prepared. Solids were kept constant at 40% by addition of water, as
necessary. The epoxysilane stirred in readily, without any apparent shock to
the
latex. The addt:ct preparations remained fluid, without sludge or apparent
viscosity buildt:p, for at least several weeks. When the formulations were 24
hours
old, they were applied to cherry veneer using a draw down bar (two coats). The
same controls as for Example 1 were used in this Example. The films were aged
3
days at 60 °C iri order to simulate an extended room temperature cure.
The films
were then tested, and gave the following results.
Latex A Latex A + Latex A +
O.Seq l.Oeq
Alone Diepoxy Diepoxy Latex B
16 hour spot test:
Water 10 10 10 10
1 % Dreft cteEecgent5 9 10 5
Vinegar 8 10 10 9
1 hour spot test:
50% EtOF-I 7 9 10 2
3A EtOH 1 7 8-9 0
7% Ammonia 0 6 9 1
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As with Example 1, Latex A alone performed well; however, the addition of
the diepoxy considerably boosted its performance, to levels far surpassing
that
achieved from the Latex B film.
