BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to (meth)acrylic latex copolymers. As employed herein, the terminology “(meth)acrylic” is intended to mean “acrylic or methacrylic”. More particularly, the present invention relates to an aqueous (meth)acrylic latex copolymer, modified by incorporation of a vinyl silane bearing hydrolyzable groups, such as alkoxy groups, which cures at room temperature without added catalyst after application to a substrate to provide a crosslinked, solvent resistant film or object, wherein said copolymer is stable for at least one year at room temperature.
2. Description of Related Art
There is extensive prior art on free radical copolymerization of unsaturated silanes with organic comonomers, in solvent-borne systems as well as in waterborne systems. Although not completely absent, shelf life problems and problems due to high levels of silane incorporation are not of overriding importance in non-aqueous systems. In waterborne systems, most of the art ignores shelf life issues, or does not attempt to provide long shelf lives, particularly when higher concentrations of silanes are involved. For specialized applications, these systems can be used shortly after synthesis.
Aqueous dispersion polymers (commonly, latex, latexes, latices) are well known. Some general references include:
Waterborne and Solvent Based Acrylics and their End user Applications, ed. P. Oldring and P. Lam, Volume I of Surface Coatings Technology, John Wiley and Sons, New York, 1997, especially chapter II, and;
Resins for Surface Coatings, Volume 1, Acrylics and Epoxies, H. Coyard, P. Deligny and N. Tuck, John Wiley and Sons, New York, 2001.
In these references typical synthesis conditions, initiator techniques, comonomers, end use properties and application conditions are described.
Latexes can be provided with superior properties for use in coatings, sealants, and adhesives by incorporation of organofunctional alkoxy silanes in the polymer. The superior properties include resistance to common household chemicals and to solvents, as well as resistance of latex paints to scrubbing with household cleaning agents. In sealants, a sealant that can be obtained that produces joints that are resistant to the environment, are flexible, and do not flow after curing in place. The properties arise from the crosslinking of the polymer chains in the latex after application and flow out or coalescence of the latex. Inclusion of silanes provides an effective mechanism for creating “self-crosslinkable” latex polymers, which do not need the addition of a separate crosslinking agent—that is, they are “one pack” systems, not “two pack” systems. There are also chemistries that do not involve silicon-containing comonomers that achieve some of the benefits of one pack, self-crosslinkable latex systems. Silicon (silane) based technologies offer superior resistance to degradation by UV light and the environment, compared to most other technologies.
This technology—based on alkoxysilane comonomers—has been practiced to some degree for years in a limited number of latex applications. However, there are deficiencies in what has been achieved to date, particularly with regard to combining good stability and good low temperature cure.
First—alkoxy silanes are reactive with water. Hydrolysis of the alkoxy groups attached to silicon, such as methoxy or ethoxy groups, occurs readily and produces free alcohol, such as methanol or ethanol. Remaining on the silicon atom after hydrolysis is an —OH group, viz., a silanol. The condensation of two silanols to form an Si—O—Si bond, with the release of water, is thermodynamically favored. Unfortunately, premature hydrolysis and condensation can destroy a silane and make a siloxane polymer of it before it has a chance to be incorporated into a latex in a uniform and well controlled fashion during polymerization. Hydrolysis and condensation after incorporation of the silane can prematurely crosslink the latex polymers during storage, resulting in solidification and gelation of the latex or latex-containing product in the container. If the crosslinking occurs within the latex microparticles, gelation may not be apparent, but the particles will not flow together and will not coalesce after application. This can result in reduced gloss (for coatings) or brittle films that have no integrity when exposed to solvents, or sealants with poor integrity. On the other hand, if this process can be controlled, a latex can be produced which uses this chemistry to crosslink the polymer system after application, to give superior properties.
In some applications, it is possible to heat the substrate after application of a silane-containing latex coating. The “stoving” or baking of articles coated with paint is well known. This heat can be used to “activate” the silane chemistry described, provided the chemistry can be kept “latent” while the silane-modified polymer system is stored on the shelf awaiting use. However, heating uses energy and some substrates may not be able to withstand heating. Catalysts, such as acids, bases, and metallic compounds (tins, titanium derivatives, etc.) may be used to catalyze the reaction. This is normally accomplished by using a two-pack system, which is less desirable than a one-pack system. Two-pack systems require control of the amount of additive, and may have very limited “open life” or “pot life” after addition of the catalytic agent.
Thus, it is particularly desirable to have a one-pack system, which cures under ambient conditions after application, and which, at the same time, does not prematurely react during storage. For practical use, a product such as a coating must be stable during storage for many months or years. This is an extremely difficult goal to achieve, owing to the conflicting needs of reactivity and stability. Any approach that relies on the use of an extremely unreactive silane that can survive storage because of its low reactivity faces the problem of to how make the unreactive silane become reactive on command. To achieve this without heat or a catalyst is very difficult.
In some cases, the goal of shelf stability and room temperature cure in a one-pack system can be achieved by using extremely low concentrations of silanes. The rate of condensation of two silanols to form a siloxane crosslink is proportional to the square of the concentration of silanol groups. (The rate equation is second order in silanol concentration.) Thus, the condensation reaction can be slowed by reducing the silane concentration, and the effect is very strong because of the dependence on the square of the concentration. However, if one wishes to obtain a higher level of properties and faster cure of the system after application, it is desirable to increase the silane concentration above levels that are typically stable through the use of very low silane concentrations, i.e., above small fractions of one weight percent in the polymer.
As can be seen from these comments, trying to control the chemistry occurring in a latex polymer system is not simple or straightforward. Factors which can influence the results are:
1. Temperature. Polymerizations are typically carried out at elevated temperatures, such as 60 to 65 degrees Celsius. Storage may be at room temperature. Application is usually at or near room temperature, but the applied coating may be heated.
2. Water concentration. While water concentration is high in the aqueous phase—nearly 55 moles per liter—it will be much less in the oil phase. Hydrolysis and condensation rates are influenced by water concentration.
3. Solubility. A hydrolyzed silane, carrying silanols, is much more water soluble than the unhydrolyzed silane. A vinyl silane has a different ratio of polar and non-polar groups than a silane with a methacryloxypropyl substituent on silicon.
4. Chemical structure. Monomeric vinyl silanes tend to be more reactive to hydrolysis than silanes with the same alkoxy groups in which the silicon is not directly attached to a vinyl (unsaturated) group. Once polymerized into an organic copolymer, the alkoxy groups on silicon that is derived from a vinyl silane, and that is, in turn, directly on the polymer backbone, will have reduced reactivity due to steric shielding by the bulky polymer chain. The same factor reduces reactivity for condensation as well as for hydrolysis. In comparison, the silicon derived from a methacryloxypropyl silane is several atoms away from the backbone, and its chemistry is less influenced by steric factors.
5. Environmental variables. Factors, such as pH and the concentration of acidic or basic groups or metal ions and nucleophiles in the reactants, will influence the silane chemistry in different ways, depending on the type of silane, whether hydrolysis and/or condensation are being considered, and the like.
The complexity of these interactions makes it extremely difficult to predict the results of a synthesis before actually running the reaction and testing the results.
Commercially available silanes that can copolymerize by free radical induced addition polymerization with acrylic and vinyl organic comonomers, and that are available in sufficiently large production quantities to be practical for large scale industrial use, are either vinyl functional silanes or methacrylate functional silanes. An example of a vinyl functional silane is vinyltrimethoxysilane. An example of a methacrylate functional silane is methacryloxypropyltrimethoxysilane. As a class, vinyl functional silanes are substantially less expensive per pound than methacrylate silanes. For large scale industrial applications, therefore, the use of vinyl functional silanes is preferred on a cost per alkoxy silane (or per silanol) group. In fact, both the lower cost per pound of vinyl silanes (relative to methacrylate silanes) as well as their lower equivalent weight per alkoxy silane group, may be necessary to achieve a commercially viable and economically feasible technology. Owing to the selective nature of the reactivity of the double bond during copolymerization with vinyl and methacrylic or acrylic monomers, vinyl silanes can copolymerize readily with vinyl monomers, such as vinyl acetate. Vinyl monomers do not readily copolymerize with (meth)acrylate double bonds and special care is required to achieve uniform incorporation of any vinyl monomer (whether it is a silane or not) into polymers consisting primarily of methacrylate or styrenic monomers. However, for many end uses, more expensive methacrylate or acrylate organic comonomers are preferred, because the resulting polymers have superior durabilty, weatherabilty, higher glass transition temperatures, and other superior properties, even in the absence of silane comonomers.
When considering the rate of reaction to incorporate the silane into the polymer, one must also consider the rate of hydrolysis of the silane before and after incorporation in the latex polymer, as noted above. In general, trialkoxyvinylsilanes hydrolyze more quickly than (meth)acryloxyalkyltrialkoxysilanes as free monomers. Once the silanes have been incorporated into the polymer, the trialkoxy silane residue will tend to be less reactive in hydrolysis and condensation because of the steric shielding arising from the location of the silicon directly on the polymer backbone.
Thus, overall, the problem to be solved is how to achieve a shelf stable, one-pack, silane modified aqueous dispersion polymer (latex) system using vinyl silanes with (meth)acrylic organic comonomers, while achieving silane concentrations well above 1% by weight, up to 5% or even more, that will cure at room temperature to a solvent and chemical resistant product, and that can be stored for at least six months and, preferably, over one year, without premature crosslinking to a degree sufficient to render them substantially useless for coatings applications.
U.K. Patent No. 1,407,827 discloses a process for the manufacture of stable coagulate-free aqueous vinyl dispersions having improved adhesion. In this process, (i) (a) one or more monomers selected from vinyl esters of carboxylic acids, acrylic acid esters, and methacrylic acid esters, and optionally up to 25% by weight (relative to the total weight of component (i)) of one or more other singly-olefinically-unsaturated water-insoluble monomers, or (b) a mixture of styrene and up to 40% by weight (relative to the mixtue) of butadiene, is copolymerized with (ii) from 0.3 to 5% by weight (relative to the total weight of component (i)) of a silicon compound of a given general formula. Polymerization is carried out at a temperature within the range of from −15 to +100° C. in an aqueous phase, and in the presence of a water-soluble free-radical initiator and an emulsifier and/or protective colloid.
U.S. Pat. No. 3,575,910 discloses silicone-acrylate copolymers, aqueous emulsions of these copolymers, latex paints containing the copolymers and articles of manufacture having a coating containing the copolymers.
U.S. Pat. No. 3,706,697 discloses that the aqueous emulsion polymerization of acryloxyalkyl alkoxysilane, alkyl acrylic esters, and optionally other vinyl monomers produces copolymers that are curable at low temperatures. The silane may be introduced to the polymerization after a portion of the other monomers are polymerized. It is said that heat curing improves the solvent resistance of cured as-cast films of the latex and that silanol curing catalysts enhance the cure rate.
U.S. Pat. Nos. 3,729,438 and 3,814,716 disclose latex polymers comprising a dispersion of an interpolymer selected from the class consisting of (A) a copolymer of vinyl acetate and vinyl hydrolyzable silane and (B) a terpolymer of vinyl acetate, an ester, e.g., acrylic ester, maleic ester or fumarate ester, and vinyl hydrolyzable silane, as well as the crosslinked polymers derived therefrom. The latex polymers are said to have utility as protective surface coatings and as vehicles for paint formulations.
U.S. Pat. No. 4,716,194 discloses that the removability of acrylate based pressure sensitive adhesives is substantially improved by the addition thereto of a small amount of an organofunctional silane monomer.
U.S. Pat. No. 5,214,095 discloses stable, aqueous emulsion copolymers with controllable siloxane crosslinking functionality. These copolymers are prepared by a concurrent free radical and cationic initiated emulsion polymerization of at least one free radical initiatable monomer, at least one linear siloxane precursor monomer, and at least one bifunctional silane monomer having both free radical polymerizable and silicon functional groups. The copolymers are said to be useful in curable coatings, paints, caulks, adhesives, non-woven and ceramic compositions and as modifiers, processing aids and additives in thermoplastics, cements and asphalts.
U.S. Pat. No. 5,482,994 discloses polymer latices that are compositions formed by adding an unsaturated alkoxy silane and an initiator to a preformed emulsion polymer. The polymer latices are said to have utility as protective surface coatings, adhesives, sealants and as vehicles for paint formulations.
U.S. Pat. No. 5,599,597 discloses unreinforced or reinforced concrete moldings, for example concrete pipes, with improved corrosion resistance to acids and acidic sewage, improved permeation resistance to inorganic and organic liquids and gases and improved mechanical stability, produced by molding with machines, for example in press molding machines or extrusion machines or concrete pipe pressing machines, and allowing to harden plastic-viscous concrete mixtures of hydraulic inorganic binders, preferably cement, aggregates and water, where, in the preparation of the plastic-viscous concrete mixtures, to the latter has been added in a positive mixer an effective amount of an aqueous plastics dispersion based on anionic and hydrolysis-resistant copolymers of ethylenically unsaturated monomers, the minimum film forming temperature (MFT) of which is above the setting temperature of the concrete mixture, preferably above 23° C.
U.S. Pat. No. 5,932,651 discloses emulsion copolymerizing a particular crosslinker, i.e., either a siloxane or silazane, with an organic monomer. An emulsion can be formed having particles consisting of polymer chains formed from organic monomer. Depending on the crosslinker and reaction conditions, these emulsion polymer chains can be either crosslinked or uncrosslinked. The uncrosslinked polymer chains can be crosslinked at a later point by the addition of a suitable catalyst.
U.S. Pat. No. 5,994,428 discloses storage-stable, silane-modified core-shell copolymers comprising a shell-forming copolymer 1 of a) from 70 to 95% by weight, based on the overall weight of the shell, of acrylic and/or methacrylic C1- to C10-alkyl esters of which from 20 to 80% by weight have a water solubility of not more than 2 g/l and from 80 to 20% by weight, based in each case on the comonomers a), have a water solubility of at least 10 g/l, and b) from 5 to 30% by weight, based on the overall weight of the shell, of one or more ethylenically unsaturated, functional and water-soluble monomers including a proportion of from 25 to 100% by weight, based on the comonomers b), of unsaturated carboxylic acids, and a core-forming copolymer II of one or more monomers c) from the group of the vinyl esters, monoolefinically unsaturated mono- or dicarboxylic esters, vinylaromatic compounds, olefins, 1,3-dienes and vinyl halides, wherein the shell contains no silane compounds and the core comprises one or more silane compounds d) from the group of the mercaptosilanes alone or in combination with olefinically unsaturated, hydrolyzable silicon compounds.
U.S. Pat. No. 6,130,287 discloses an emulsion polymer comprising a protective colloid and a functionalized silane component which is of a given structural formula.
WO 98/35994 discloses emulsion polymers that are said to have an excellent combination of blocking resistance, water spotting resistance and ethanol spotting resistance. These polymers are made from a monomer mixture including a monomer with a highly polar group that includes either a carboxylated or sulfonated monomer, or both, a monomer having a hydrolyzable silicone group, and a nonfunctional monomer that can be selected to provide a desired minimum film formation temperature. These polymers are said to be useful in paint and coatings applications.
European Patent Publication No. 0 327 376 discloses copolymers of vinyl esters and silicon monomers, with very low levels of the silicon monomer, that are said to be especially suitable as binders for emulsion paints, giving good scrub resistance. Vinyltrimethoxysilane is copolymerized with organic comonomers comprising at least 40% vinyl acetate. Substantial or full hydrolysis of the silanes to silanols is expected. pH is not mentioned as a critical variable, and no pH ranges are indicated.
Bourne et al., J Coatings Technology, 54:69-82, #684, (January, 1982) describe attempts to obtain stable silane-modified latex copolymers from a variety of acrylate and methacrylate organic monomers by copolymerization with various methacrylate functional alkoxy silanes. These attempts met with failure. A range of pH conditions was attempted with ethyl acrylate as the comonomer. Conditions including starting at pH 9 or pH 7 and allowing the pH to drift, as well as pH 9 or no pH adjustment resulted in gelation (coagulation) during the reaction. Runs made at pH 7 did not coagulate during synthesis. However, even those preparations gave unacceptable levels of coagulum and inadequate shelf stability.
Marcu et al., Macromolecules, 36:328-332 (2003) carried out extensive studies using extraordinary techniques in attempts to obtain stable silane modified emulsion polymers. These authors attempted to copolymerize vinyltriethoxysilane with butyl acrylate. In order to obtain stable emulsion polymers, they had to resort to the use of a “mini-emulsion” technique. This technique involved addition of hexadecane to the reaction mixture to form an oil phase that might “protect” the silane from hydrolysis, plus the use of ultrasound to achieve extremely high shear and agitation. pH control is mentioned as being prominent in the literature and is used in their work. The experiments were run using sodium bicarbonate buffer at one mole % on monomers, at pH 6.5 (page 330, experimental section.) Even with these techniques, using the reaction scheme of batch reaction, they were unable to get the silane to copolymerize by free radical addition polymerization with the butyl acrylate. Instead, hydrolysis and condensation reactions of the reactions of vinyltriethoxysilane produced some form of oligomer, which eventually reacted with the organic latex polymer, possibly by transesterification or some other heterolytic mechanism. Control reactions run without the oil phase were also run, and gave poor results.
Cooke et al., Emulsion Polymerization with Hindered Silane Monomers, presented at Silicones in Coatings III, Barcelona, Spain, Mar. 28-30, 2000, addresses the use of highly hindered silanes with reduced hydrolytic reactivity, such as vinyl-tri-isopropoxysilane and methacryloxypropyl-tri-isopropoxysilane, with acrylate or methacrylate comonomers. Further studies with this type of silane were reported in Silicones in Coatings IV, at Guildford, UK, May 30-31, 2002. In this work, the use of sodium bicarbonate buffer to control pH is described and it is stated not to be necessary with the vinyl silanes, only with the methacrylate silanes. This work does not involve vinyltriethoxysilane.
Many other publications and patents exist, related to the objective of this work to a lesser degree than those cited above. A review is presented in Silanes in Coatings Technology, published in The Journal of the Oil and Colour Chemists' Association, 79:539-550 (December, 1996). The large number of publications and patents since the 1970's attests to the difficulty of this problem. Many give conflicting advice about conditions, such as pH and reaction conditions, and many involve other reagents, other comonomers, and the like, all of which have the potential to change the complex balance among hydrolysis, condensation, and free radical polymerization in a system with a water phase and an oil phase.
The disclosures of the foregoing are incorporated herein by reference in their entirety.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided an (meth)acrylic latex, modified with a vinyl silane, e.g., vinyltriethoxysilane, under specified conditions by a novel process and synthesized to a specific range of composition, that is stable to storage up to three years at room temperature and provides self curing latex systems that cure to solvent resistant films with no added catalyst or heat.
In one embodiment, the present invention is directed to a composition that is an aqueous (meth)acrylic latex copolymer, modified by incorporation of a vinyl silane bearing hydrolyzable groups, such as alkoxy groups, that cures at room temperature without added catalyst after application to a substrate to provide a crosslinked, solvent resistant film or object, and that is stable for at least one year at room temperature. Preferably, the latex comprises at least about 10% and up to about 60% of the alkoxy groups derived from the vinylalkoxysilane hydrolyzed to release alcohol during the polymerization. More preferably, the vinyl silane is a vinyltrialkoxysilane, where the alkoxy moiety is ethoxy or n-propoxy.
In another embodiment, the present invention is directed to a process for making this latex that is broadly similar to known latex polymerizations, but is unique in that a specific range of pH is maintained during the polymerization, the preferred vinyl silane is vinyltriethoxysilane, and the silane concentration is up to 3 mole %, possibly as much as 5 mole %, and greater than 0.5 mole % relative to other monomers. This process deliberately hydrolyzes some of the alkoxy silane groups to release alcohol and form silanols, but does not produce such a high level of silanols that the system becomes unstable and will not survive shelf aging.
In contrast to known art, which describes pH control during synthesis of silane-containing latexes in many conflicting references with many different polymer systems and silanes, and discusses avoiding silane hydrolysis and/or condensation, in this invention silane reactions of hydrolysis and further silane reactions are controlled to specific, desirable levels.
More particularly, the present invention is directed to a process for preparing a shelf-stable, one-pack, silane modified (meth)acrylic latex interpolymer composition comprising continuously adding at least a portion of a mixture comprising at least 0.5 mole percent of a vinyl silane comprising hydrolyzable groups and up to 99.5 mole percent of a (meth)acrylic monomer to water and a surfactant in a reaction vessel, wherein said addition is carried out in the presence of a polymerization initiator and buffer sufficient to maintain the pH of the reaction at a level of at least 6 throughout the reaction, while simultaneously hydrolyzing from about 10 to about 60% of the hydrolyzable groups of the vinyl silane.
In another embodiment, the present invention is directed to a shelf-stable, one-pack, silane modified (meth)acrylic latex interpolymer composition prepared by a process comprising continuously adding at least a portion of a mixture comprising at least 0.5 mole percent of a vinyl silane comprising hydrolyzable groups and up to 99.5 mole percent of a (meth)acrylic monomer to water and a surfactant in a reaction vessel, wherein said addition is carried out in the presence of a polymerization initiator and buffer sufficient to maintain the pH of the reaction at a level of at least 6 throughout the reaction, while simultaneously hydrolyzing from about 10 to about 60% of the hydrolyzable groups of the vinyl silane.