EP0963387A1

EP0963387A1 - Emulsion polymers which provide scrub resistance and blocking resistance in high and low pvc paint formulations

Info

Publication number: EP0963387A1
Application number: EP98904929A
Authority: EP
Inventors: Brigitte Emelie; Thomas Hiff; Christopher Gopsill; Ute Schuster
Original assignee: Union Carbide Corp
Current assignee: Union Carbide Corp
Priority date: 1997-02-04
Filing date: 1998-02-04
Publication date: 1999-12-15
Also published as: BR9807302A; KR20000070782A; AR011630A1; WO1998033831A1; CA2279495A1; GB9702241D0; AU6268298A

Abstract

An aqueous emulsion is made in a two-stage polymerization. In the first stage, monomers which give rise to a low Tg polymer are polymerized, whereas in the second stage, monomers which give rise to a high Tg polymer are polymerized. In one or both of the stages, a monomer having at least two carboxyl or carboxylate groups forms part of the monomer mixture. The resulting polymer is particularly useful in coating compositions, where it provides a good combination of blocking and scrub resistance.

Description

EMULSION POLYMERS WHICH PROVIDE SCRUB RESISTANCE AND BLOCKING RESISTANCE IN HIGH AND LOW PVC PAINT FORMULATIONS

This invention relates to emulsion polymers and to paints containing emulsion polymers.

Paints and coatings based on emulsion polymers are well known and generally referred to as "latex" paints or coatings. These paints and coatings are water- based, having a continuous aqueous phase in which minute polymer particles are dispersed. The polymer particles are commonly polymers of acrylate, styrenic or ethylenic monomers such as methyl methacrylate, butyl acrylate, 2-ethyl hexyl acrylate, styrene, vinyl acetate, vinyl veova, or ethylene.

Despite being water-based, latex paints and coatings nonetheless tend to contain a significant level of volatile organic compounds (VOCs). Often, this is due to the presence of an organic cosolvent or a "coalescing agent", which aids the copolymer in forming a film when the paint is dried. Due to environmental, health and odor concerns, it is desired to reduce the level of VOCs in these latex paints. One way to accomplish this is to reduce the level of solvents and coalescing agents in the paint formulation, or eliminate them entirely.

Unfortunately, removing solvent and coalescing agents from a paint formulation creates problems. One problem is that in order to make good films when no coalescing agent is present, lower T_g polymers must usually be used. Latices of low T_g polymers are known; however, their use is associated with poor blocking resistance. "Blocking" is the tendency of a film to stick to itself when stacked under pressure.

The blocking problem can be overcome to some extent by using a high extender level, but this solution is only applicable to paints such as flat interior paints which have a relatively high pigment volume concentration (PVC). Paints having high extender levels are not suitable for satin and semi-gloss applications. Furthermore, at the binder levels used in high PVC paints, scrub resistance and gloss suffer.

On the other hand, low PVC paints tend to have good scrub resistance due to their relatively high binder content, but their blocking resistance suffers because the extender level is lower. Thus, it is seen that there is often a trade-off between blocking resistance and scrub resistance, and that it has proven difficult to provide a latex which provides for both good blocking resistance and good scrub resistance in both high and low PVC paints, particularly in low VOC formulations.

It would therefore be desirable to provide a polymer latex which can be formulated into paint and other coating formulations which have both good scrub resistance and good blocking resistance. It would be particularly desirable if that latex provided those results over a wide range of pigment volume concentrations, and in formulations containing low or no VOCs.

In one aspect, this invention is an aqueous emulsion having an aqueous phase and a dispersed polymer phase, wherein at least 50 percent of the weight of the dispersed polymer phase comprises particles prepared in a multistage polymerization process. In the multistage process, a "first stage" monomer or mixture thereof is polymerized in a first stage, wherein the first stage monomer or monomer mixture polymerizes to form a polymer having a T_g of less than 25°C, and a "second stage" monomer or mixture thereof is polymerized in a second stage, wherein the second stage monomer or monomer mixture polymerizes to form a polymer having a T_gof greater than 60°C. The monomers polymerized in said first stage constitute from 50 to 95 percent of the total weight of the monomers, and the monomers polymerized in said second stage constitute from 5 to 50 weight percent of the total weight of the monomers. Incorporated into either the first stage monomer mixture or the second stage monomer mixture, or both, is 0.1 to 2 percent by weight, based on the total weight of the monomers, of an addition polymerizable ethylenically unsaturated monomer containing at least two carboxyl or carboxylate groups and 0.5 to 3 percent by weight, based on the total weight of the monomers, of an addition polymerizable, ethylenically unsaturated monomer containing a single carboxyl or carboxylate group.

In a second aspect, this invention is an aqueous emulsion having an aqueous phase and a dispersed polymer phase, wherein at least 50 percent of the weight of the dispersed polymer phase comprises particles prepared in a multistage polymerization process. In the multistage process, a "first stage" monomer or mixture thereof is polymerized in a first stage, wherein the first stage monomer or monomer mixture polymerizes to form a polymer having a T_g of less than 25°C, and a "second stage" monomer or mixture thereof is polymerized in a second stage, wherein the second stage monomer or monomer mixture polymerizes to form a polymer having a T_gof greater than 60°C. The monomers polymerized in said first stage constitute from 50 to 95 percent of the total weight of the monomers, and the monomers polymerized in said second stage constitute from 5 to 50 weight percent of the total weight of the monomers. Incorporated into either the first stage monomer mixture or the second stage monomer mixture, or both, is 0.85 to 2 percent by weight, based on the total weight of the monomers, of an addition polymerizable, ethylenically unsaturated monomer containing at least two carboxyl or carboxylate groups.

The emulsions of this invention can be formulated into paints and coatings which have good scrub resistance even when formulated into low or no VOC formulations and over a wide range of pigment volume concentrations. Typically, paints and coatings formulated with these compositions have a good combination of blocking and scrub resistance. Accordingly, another aspect of this invention is a coating comprising the aqueous emulsion of the first or second aspect of this invention.

The aqueous emulsion of this invention includes a continuous aqueous phase and a dispersed polymer phase. The dispersed polymer phase is in the form of particles of a size such that they can remain stably dispersed in the aqueous phase. A suitable size range for the particles is from 80 nm to 300 nm in diameter ("diameter" here referring to the longest dimension of the particle).

The dispersed particles comprise a polymer which is prepared in a multistaged process. In the first stage, monomers which form a polymer having a T_g of no greater than 25°C, preferably less than 10°C, more preferably between -20°C to 10°C, are polymerized.

In the second stage, monomers are polymerized which form a polymer having a T_g of at least 60°C, preferably at least 80°C. The dispersed polymer particles contain from 0.1 to 2 weight percent of repeating units formed by^' polymerizing an addition polymerizable ethylenically unsaturated monomer containing at least two carboxyl or carboxylate groups and from 0.5 to 3 weight percent of repeating units formed by polymerizing an addition polymerizable, ethylenically unsaturated monomer containing a single carboxyl or carboxylate group. These acid-containing monomers can be polymerized in either the first or second polymerization stage, or both.

In this invention, all references to T_g are those obtained by dynamical mechanical thermal analysis (DMTA) on a polymer made by pouring 15 to 20 gm of the latex into a three-inch diameter tetrafluoroetylene fluorocarbon-coated O-ring which is heated at

35°C to 40°C, drying the latex at that temperature, and then further drying it in a forced-air oven at 70°C to 80°C until the sample appears completely dry. Samples of 1 to 2 mm thickness are evaluated on a Rheometrics RDS-II spectrometer using the 7.9 mm parallel plate geometry. A frequency of 1 rad/sec is used, and the temperature range is from below the expected T_g to approximately 150°C. The percent strain is chosen so that the properties are determined in the linear viscoelastic regime.

In some cases, it may be difficult to measure the separate T_gs of the polymer. In those cases, the T_g of the polymer formed in each stage can be determined by polymerizing the monomer or monomer mixture alone in a single-stage polymerization process and measuring the T_g of the resulting polymer in the manner just described. For example, the T_g of a styrene/2-ethylhexyl acrylate mixture can be determined by polymerizing that mixture in a single-stage polymerization and measuring the T_g of the resulting polymer.

A convenient way to estimate the T_g of a polymer formed from a particular monomer mixture is through the Fox equation:

1/T_nAN = wA/T_nA + wB/T B = . . . +wN/T N

wherein A, B and N represent individual monomers in the mixture containing N monomers, wA, wB and wN represent the weight fractions of monomers A, B and N, respectively, T_gAN represents the T_g of the polymer formed, and T_gA, T_gB and T_gN represent the T_g of homopoiymers of monomers A, B and N, respectively. See T.G. Fox, Bull. Am. Physics Soc, vol. 1 (3), page 123 (1956). By using the Fox equation, it is possible to screen combinations of monomers to estimate which combinations will provide a polymer having the required T_g.

Suitable monomers which are useful for polymerization in either or both of the first and second polymerization stages include styrene, -methyl styrene, vinyl toluene, and vinyl naphthalene, t-butyl styrene, o- or p-methyl styrene and o-p-dimethyl styrene, any of which can be inertly-substituted, such as with alkyl, alkoxyl or halogen groups; nitriles such as acrylonitrile and methacrylonitrile; vinyl and vinylidene halides such as vinyl chloride and vinylidene chloride; vinyl acetate; acrylic and methacrylic esters in which the ester group is C C₂₀ alkyl, preferably C₂-C₈ alkyl, such as methyl acrylate, ethyl acrylate, n- or i-propyl acrylate, n-, t-, i- or sec-butyl acrylate, 2-ethylhexyl acrylate, cyclohexyl acrylate, isobornyl acrylate, decyl acrylate, phenyl acrylate, n-, t-, i- or sec-butyl methacrylate, ethyl methacrylate, n- or i-propyl methacrylate, 2-ethylhexyl methacrylate, methyl methacrylate, cyclohexyl methacrylate, isobornyl methacrylate, decyl methacrylate, or phenyl methacrylate. It is an advantage of this invention that an emulsion polymer having desirable properties can be made using those of the foregoing monomers which are commonly available and, in most cases, relatively inexpensive. Those having halogen groups are disfavored because they are sometimes considered to be less environmentally friendly.

In the art, many of these monomers are commonly characterized as "hard" or "soft" monomers according to the T_g of their homopolymers. Accordingly, it is well known to select a monomer or mixture of monomers for each of the polymerization stages in order to obtain, for each stage, the required T_g polymers. For example, the vinyl aromatics tend to form higher T_g polymers. By mixing "hard" and "soft" monomers in various proportions, it is possible to tailor the T_g of the resulting copolymer within a desired range.

Included in either or both of the polymerization stages is at least one monomer having at least two carboxyl or carboxylate groups. This monomer constitutes from 0.1 to 2 percent, preferably from 0.25 to 1 of the total weight of all monomers. However, when no monomer containing a single carboxyl or carboxylate group is also present, then this monomer containing multiple carboxyl or carboxylate groups constitutes at least 0.85, preferably at least 0.9 percent of the total weight of all monomers. Suitable such monomers include itaconic acid, maleic acid, fumaric acid, and succinic acid, and the corresponding carboxylates. When carboxylates are used, the counter-ion is preferably a monovalent cation, preferably an alkali metal or a fugitive base, as described below.

One or both of the monomer mixtures advantageously and preferably contains one or more addition polymerizable, ethylenically unsaturated monomers containing a single carboxyl or carboxylate group. Acrylic acid and methacrylic acid are examples of these. These monomers provide for improved paint stability when small quantities (that is,. 0.5 to 3 weight percent of all monomers) are used in conjunction with the monomer having multiple carboxyl or carboxylate groups. When these monomers are present, a lesser quantity of the monomer containing multiple carboxyl or carboxylate groups can be used.

In addition to the foregoing monomers, either of the monomer mixtures may contain one or more functional monomers such as ethyleneureamethacrylate (Nourycryl® MA-123-50, a product of Akzo Chemie or Plex 6844-0, a product of Rohm & Haas). Other monomers of the same type include methacrylamido ethyleneurea, which is sold by Rhone- Poulenc under the trade name Sipomer WAMII. Another useful functional monomer is sold under the trade name Sipomer WAM, and contains an alkyl group, an amine functionality, and a ureido ring.

It is preferred that the monomers used in the first polymerization stage contain a predominant amount (that is, more than 50 weight percent) of an alkyl acrylate having one or more, preferably four or more carbons in the alkyl group, such as methyl acrylate, n-butyl acrylate or hexyl acrylate. Quantities of a "hard" monomer, preferably styrene, methyl acrylate, or acrylonitrile, may be copolymerized with the "soft" monomer in the first polymerization stage, provided that the T_g of the resulting copolymer is within the aforementioned ranges. In the second stage, it is preferred that the monomer be predominantly (that is, more than 50 weight percent) a vinyl aromatic, preferably styrene; acrylonitrile; methyl methacrylate; or a mixture of two or more thereof. A quantity of a "soft" monomer may be employed in the second stage monomer mixture, provided that the T_g requirement is met.

The first stage monomer or mixture of monomers constitutes from 50 to 95, preferably from 60 to 95, more preferably from 60 to 0 percent of the weight of all monomers used in the preparation of the multistage particles. The second stage monomer or mixture of monomers constitutes from 5 to 50, preferably 5 to 40, more preferably 20 to 40 percent of the total weight of all monomers.

The emulsion of this invention is conveniently prepared by polymerizing the monomers just described in a two-stage emulsion polymerization in an aqueous phase. Suitable polymerization methods are well known and are described, for example, in Emulsions: Theory and Practice, by P. Becher Reinhold, New York (1959), High Polymer Latices, by D. C. Blackley, Pamerton Publishing Co., New York (1966); and Emulsion Polymer Technology, by Robert D. Athey, Jr. Marcel Dekker, Inc., New York (1991 ).

In general, the emulsion polymerization process includes adding the first stage monomers into a continuous aqueous phase under agitation. One or more surfactants are typically present in order to help form a stable emulsion having discrete droplets of desirable size. A free radical initiator or redox catalyst is usually employed to provide a commercially acceptable rate of polymerization and a high conversion of monomers to polymer. The first stage monomers may be added to the aqueous phase all at once in a batchwise operation, or all or a portion may be added continuously or in increments as the polymerization proceeds.

The second stage monomers are added after the first stage monomers are added and are generally polymerized to at least 70 weight percent conversion, preferably at least 80 weight percent conversion.

It is important to note that although the terms "first" and "second" stage polymerization are used herein, those terms are not intended to imply that the stages are conducted in any particular way. What is important is that the monomers(s) being polymerized at the end of the polymerization process give rise to a high T_g polymer, while those used earlier in the process give rise to a low T_g polymer. Thus, for example, the monomers may be added continuously to the reaction vessel (under polymerization conditions), with the transition from the first stage to the second being marked by a change in monomer composition (or ratios of monomers). The change in monomer composition or ratios may itself be abrupt or gradual. The change in monomer composition may be due to the addition of a monomer stream, such as in the case where the first stage monomer includes n-butyl acrylate, and the second stage monomer includes a styrene/n-butyl acrylate mixture. In that instance, the n-butyl acrylate may be added without interruption throughout the first and second stages, and the beginning of the second stage is marked by the commencement of a styrene stream.

Similarly, the change in monomer composition may be due to the discontinuance of a monomer stream, such as in the case where the first stage monomers include a mixture of n-butyl acrylate and styrene and the second stage monomer includes styrene, but not n-butyl acrylate. The second stage begins when the n-butyl acrylate stream ends.

Yet a third manner in which the first and second stages are separated is by varying the ratios of the monomers in the streams. For example, a styrene/butyl acrylate mixture rich in n-butyl acrylate may be added in the first stage, and the second stage can be begun simply by increasing the amount of styrene in the stream, relative to the amount of n- butyl acrylate. Of course, the first and second stages can be separated by a complete discontinuation of the first stage monomer followed by the start of an entirely different monomer stream. Yet another way to conduct the polymerization in stages is to feed all of the first stage monomers at once, followed by polymerization to a conversion of at least 70 percent, followed by the addition of the second stage monomers, either at once or gradually.

It is within the scope of this invention to use seed particles at the start of the polymerization. When seed particles are used, their weight is preferably no greater than 2 percent than the weight of the monomer mixture; in such a case, the composition of the seed particles is ignored when calculating the amounts of the various monomer types in the monomer mixture.

In an illustrative polymerization process, water, surfactant and optional seed particles are initially charged to a suitable reactor and heated to the desired polymerization temperature. The desired polymerization temperature depends on the particular catalyst and monomers employed, and typically ranges from 30°C to 100°C, preferably from 50°C to 100°C, more preferably from 60°C to 100°C. Higher temperatures can be used under special conditions, such as the use of superatmospheric pressure.

The initial charge to the reactor may include all or a portion of the first stage monomer mixture, which may include the monomer having multiple carboxyl or carboxylate groups as well as the monomer containing a single carboxyl or carboxylate group.

After the initial charge is heated to the desired temperature, one or more streams is fed to the reactor. One of those streams contains the free radical initiator. Any portion of the first stage monomer mixture which is not added to the initial charge may be added in one or more separate streams. The monomers may be pre-emulsified, but it is not critical to do so.

Additional surfactant may also be added, either as a separate stream or mixed with the catalyst or one or more of the monomers. When a mixture of monomers is polymerized in the first stage, the monomers can be added as two or more separate streams.

Following the addition of the first stage monomer(s), the reactor contents are typically heated for a period to polymerize the monomers to a conversion of at least 70 weight percent, preferably at least 80 weight percent. Afterwards, the second stage monomer(s) are added. Again, this second stage monomer mixture may contain all or a portion of the monomer having multiple carboxyl or carboxylate groups as well as the monomer containing a single carboxyl or carboxylate group. This may be done all at once, or the second stage monomer mixture may be added continuously or intermittently over a period of time. During the addition of the second stage monomer mixture, additional surfactant and/or free radical initiator may also be fed into the reaction vessel. Following addition of all monomers, the reactor contents are typically heated for a period to complete polymerization. Often, this post addition heating is conducted at a higher temperature than the main polymerization, preferably 80°C to 100°C.

The amount of monomers added and polymerized is selected so that the resulting polymer emulsion has a desired solids content, and the copolymer particles have a desired size. Preferably, the resulting emulsion has a solids content from 10 to 70 percent by weight, more preferably from 40 to 55 percent by weight, and the copolymer particles have a volume average diameter from 80 nm to 300 nm.

Any surfactant which helps to stabilize the monomer mixture as discrete droplets and the subsequent copolymer as discrete particles in the aqueous phase can be used. The surfactant may be of the nonionic, anionic or amphoteric type. Exemplary surfactants include alkali metal alkyl carboxylates, polyoxyethylene alkyl phenols, linear alkyl sulfonates, alkyl aryl sulfonates, alkylated sulfosuccinates, C₆-C₂₀ amine oxides, or N,N- bis(carboxyl alkyl) C₆-C₂₀ alkyl amines. Sodium dodecyl benzene sulfonate, sodium lauryl sulfate, disodium dodecyldiphenylether disulfone, N-octadecyl disodium sulfosuccinate, dioctyl sodium sulfosuccinate, N,N-bis-carboxyethyl lauramine, and sulfonated alkylated phenyl ethers such as DOWFAX^* 2EP and DOWFAX^* 2A1 (Trademarks of The Dow Chemical Company and both products are available from The Dow Chemical Company) are specific examples of useful surfactants. The surfactant is advantageously used in an amount from 0.1 to 2 percent, preferably from 0.1 to 0.5 percent, based on the total weight of the monomers.

Suitable free radical initiators include peroxy compounds such as peroxydisulfates (commonly known as persulfates), perphosphates, t-butyl hydroperoxide, 2,2-azobis-isobutyronitrile, cumene hydroperoxide and hydrogen peroxide. Ammonium persulfate, sodium persulfate and potassium persulfate are preferred initiators. Redox catalysts, which are activated in the water phase through a water-soluble reducing agent can also be used. For example, the free radical initiators just mentioned can be used combined with sodium formaldehyde sulfoxylate, sodium bisulfite, ascorbic acid or ferric nitrate. A hydrogen peroxide/ferric nitrate mixture has been found to provide the polymer with excellent scrub resistance in some instances. The free radical initiator is advantageously used in an amount from 0.01 to 5 percent, preferably 0.1 to 2 percent, based on the weight of the monomers. If desired, an additional amount of catalyst in excess of the foregoing amounts may be added after the addition of the monomer streams in order to finish off the polymerization.

It is preferred to stabilize the resulting polymer emulsion by adjusting the pH to above 7.0, preferably from 7.5 to 11 , more preferably from 7.8 to 9.5. This can be done by adding a fugitive base, for example, ammonia, dimethylamine, diethyl amine, aminopropanol, ammonium hydroxide or 2-amino-2-methyl-1 -propanol. However, because of the desire to use the polymer emulsion in low or no VOC coating formulations, it is preferred to stabilize the emulsion through the addition of an alkali such as sodium hydroxide, potassium hydroxide or sodium or potassium carbonate. This base may be added towards the end of the addition of the monomer stream(s), after all the monomer addition has been completed, or after the polymerization reaction is finished. The carbonates may be present throughout the polymerization.

Other ingredients can also be used during the polymerization process as desired, such as chain transfer agents, buffers, or preservatives. Post-additions of these ingredients, or others which may be added for a specific purpose, can also be made.

Following the polymerization, the resulting emulsion may be steam stripped or otherwise treated to remove impurities and unreacted monomers.

In some instances, paints and coatings formulated from the emulsion of this invention may tend to crack slightly, particularly when applied at low temperatures on porous surfaces. This problem is easily overcome by blending it with a small quantity of a second emulsion polymer in which the polymer particles have a T_g of below 15°C, preferably below 10°C. The second emulsion can be prepared in a simple, single-stage polymerization, but may also be prepared in a multistage polymerization if the monomers used in each stage polymerize to form a polymer having a T_g as just described. The monomers used in the second emulsion may be the same ones mentioned above with respect to the emulsion of this invention, provided that they are selected together as is known in the art to form a polymer having the requisite T . Polymers containing a predominant amount of alkyl acrylates are preferred. Particular polymers include copolymers of styrene and n-hexyl acrylate; styrene and n-butyl acrylate; styrene and 2-ethylhexyl acrylate. These copolymers may be modified with small amounts of a monomer having a carboxyl or carboxylate group, or a monomer having a sulfonate group. The emulsion of this invention and the second emulsion are advantageously mixed in proportions by weight of from 85:15 to 98:2 on a solids basis, preferably from 90:10 to 97:3.

The emulsion of this invention can be formulated into a variety of paints and coatings.

In general, it can be used in the same manner as conventional polymer emulsions to formulate paints and coatings, and the various ingredients that are conventionally used in such paints and coatings can be used in conjunction with this invention. The science of coating formulation is well known in the art, and described, for example in "Handbook of Coatings Additives" (ed. Leonard J. Calbo, Dekker, 1986).

For example, the formulation will generally contain a filler, opacifying agent or pigment, such as calcium carbonate, talc, silica, aluminum hydroxide, glass powder, titanium dioxide, red lead, zinc chromate, zinc phosphate, calcium molybdate, barium borate, red oxide, carbon black, Hansa Yellow, Benzidine Yellow, Phthalocyanine Blue, or Quinacridone Red. These are generally used in amounts sufficient to provide the coating with a PVC of from 30 to 80 percent. It is an advantage of this invention that it can be formulated into a variety of paints having a wide range of pigment volume concentrations (PVC's) which have good blocking and scrubbing resistance as well as other useful properties.

In addition, the formulation may contain inorganic dispersants such as sodium hexametaphosphate or sodium tripolyphosphate, organic dispersants such as the polycarboxylic acid polymers (for example, Nopcoperse™ 44c, from Summopco Co., Ltd. and Orotan™ 681 , from Rohm & Haas); wetting agents; thickeners, such as polyvinyl alcohols, polyurethanes such as Acrylsol RM8, from Rohm & Haas, and cellulosic derivatives; crosslinking agents such as water-soluble polyvalent metal salts, aziridine compounds, water-soluble epoxy or melamine resins, or water-dispersible blocked isocyanates; surfactants; matting agents and defoamers. In addition, solvents may be added, such as alcohols, glycol ethers, hydroxy-tertiary amines, ketoximes, active methylene compounds and lactams; and coalescing agents may be used. However, it is an advantage of this invention that the use of coalescing type solvents are not necessary in order to obtain good film formation.

An important advantage of this invention is that it provides for excellent paint properties, particularly blocking and scrub resistance, even when formulated into low VOC paints. Blocking resistance can be measured in different ways under different conditions. One measure of blocking resistance is the so-called Byk blocking resistance test, using a Byk blocking tester. In this test, 150 μm (wet thickness) films are drawn down on opacity charts or Leneta foil and dried at room temperature for 24 hours. 25 by 75 mm strips of film are cut and glued to the blocking tester's object carrier using joining tape. Two mounted films are placed in contact at an angle of 90 degrees to give an area of contact of 625 mm². A 5 kg weight is applied for a predetermined period of time, and removed. The blocked specimens are then stored 15 minutes without load before operating the blocking tester to evaluate the deblocking force. Paints formulated according to this invention often exhibit a separation force of less than 1.75 N/cm² after being pressed together at ambient temperature and humidity for 4 days.

Scrub resistance is conveniently measured according to ASTM 2486 or DIN 53778, after 7 or 28 days aging time. Resistances according to this test commonly exceed 640 strokes, preferably 700 strokes, more preferably 1000 strokes, with a weight loss of no greater than 0.506, preferably no greater than 0.45 grams. Scrubbing resistance is determined as the average of two samples, whose individual values do not vary by more than 25 percent. Weight loss is determined according to DIN 53778.

Thus, although the paint formulation may contain organic cosolvents and coalescents, it is preferred that these be minimized and most preferred that they be eliminated. The preferred paint formulation will contain less than 8 percent by weight, more preferably less than 3 percent by weight, and most preferably essentially none of an organic cosolvent and less than 8 percent by weight, more preferably less than 3 percent by weight and most preferably essentially none of a coalescent.

The following examples are provided to illustrate the invention, but are not intended to limit its scope. All parts and percentages are by weight unless otherwise indicated. Examples 1 to 4

An emulsion polymer (Latex Example 1 ) was prepared as follows:

To a suitable reactor was added an initial charge of 70.129 parts water, 0.806 part of a polystyrene latex), 0.75 part of itaconic acid and 0.01 part of a 40 percent solution of VERSENOL^* 120 (Trademark of The Dow Chemical Company) brand chelating agent in water. The reactor contents were heated to 80°C. Three feed streams were begun at the same time (t=0). The first stream was an aqueous stream added over 360 minutes containing 0.4 part sodium persulfate. The second stream was an aqueous stream added over 360 minutes containing 2 parts of a 30 percent solution of Aerosol A-102 surfactant in water. The third stream contained 47 parts butyl acrylate and 22.8 parts styrene, and was added over 200 minutes. The temperature of the reactor contents was maintained at 80°C throughout the addition of these streams.

Beginning at time = 200 minutes, a fourth stream of 29.45 parts styrene and 1 part of acrylic acid was added over a period of 100 minutes, an aqueous stream containing 0.2 part sodium persulfate was added over the course of an hour and an aqueous stream containing 0.1 part ascorbic acid was added over a period of 20 minutes. The temperature of the reactor contents was thereafter increased to 90°C for 10 minutes. The latex was neutralized with potassium hydroxide. The product (Latex Example 1 ) had a particle size of approximately 140 nm and contained 48 percent solids.

The T_g of the resulting polymer is evaluated by DMTA using the method described before. The T_g of the polymer prepared in the first stage (which includes the itaconic acid provided in the initial charge) is 15° to 25°C; that of the second stage polymer was 90°C.

The latex was then blended in a 90:10 weight ratio (solids basis) with a second latex containing styrene/n-butyl acrylate copolymer particles which were prepared in a single-stage polymerization. The blended latex was designated Blended Latex Example 1.

Latex Example 2 and Blended Latex Example 2 were prepared in the same manner, except the itaconic acid was increased to 1 part, and the acrylic acid was eliminated. The T_g values of Latex Example 2 are similar to those of Latex Example 1. Latex Example 3 and Blended Latex Example 3 was prepared in the same manner as Latex Example 1 and Blended Latex Example 1 , except that only 0.5 part itaconic acid was used, 0.5 part of fumaric acid wasadded in the initial charge to the reactor, and the acrylic acid was eliminated. The T_g values of Latex Example 3 are similar to those of Latex Example 1.

Latex Example 4 and Blended Latex Example 4 is prepared in the same manner as Latex Example 1 and Blended Latex Example 1 , except the acrylic acid is reduced to 0.5 part. The T_g values of Latex Example 4 are similar to those of Latex Example 1.

Blended Latex Examples 1 to 4 were formulated into paints using a formulation as described in Table I. The paints were designated as Paint Samples 1 to 4, respectively. All have a PVC of 39 percent.

TABLE I

Ingredient Parts by Weight Water 51.08

5% Hexametaphosphate 31 .50 solution Natrosol Plus 330¹ 3.00

Coatex P50² 4.01

Mergal K6N³ 1 .81

Aginian 232M⁴ 3.01

Tiona 535⁵ 219.10

Durcal 2⁶ 75.32

Water 8.22

Blended Latex (46% solids) 477.00

Ropaque OP-62⁷ 70.1 1

Agitan 232M 2.00

Neodol 91-6⁸ 4.01

Acrysol TT-935⁹ 4.50 Water 40.83

¹A hydrophobically modified hydroxyethyl cellulose thickener sold by Aqualon; ²A 40% active polyacrylate dispersant sold by Coatex; ³A preservative sold by Riedel de Haen; A defoamer sold by Muenzing;

⁵A grade of titanium dioxide sold by SCM; ⁶A calcium carbonate filler sold by Pluess Stauffer; ⁷ A 37.5% solids opaque polymer sold by Rohm & Haas; ⁸A nonionic surfactant sold by Shell; ⁹A thickener sold by Rohm & Haas, 35% active.

Each paint was evaluated for minimum milm mormation temperature (MFFT) by casting a 150 μm film of the emulsion on a heating plate that had a temperature gradient. The film was dried and the minimum temperature at which a coherent film was formed was recorded as the MFFT. The Brookfield viscosity of the paints was measured after formulating and again after 28 days. Gloss was measured at 60° and 85° using a Byk color glossmeter on a 150 μm (wet thickness) film that was dried for one day at ambient conditions. The one-day room temperature blocking of this paint formulation was evaluated by drawing duplicate 150 μm (wet thickness) films on Leneta foil and drying the films for 1 day at room temperature. The films were then placed face-to-face and either a 0.1 metric ton/square meter pressure or a 0.5 metric ton/square meter pressure was applied for 24 hours at room temperature. The films were then separated, with the force required to separate them being rated on a scale of 0 to 5, with 5 indicating no blocking (the films fall apart without added force) and 0 indicating that the films cannot be pulled apart without destroying them. The percentage of damage to the film was also observed and reported. Scrub resistance was evaluated according to ASTM 2486 after 7 days and again after 28 days; here, higher values indicated superior performance. This test was performed twice for each dried coating, and both results are reported. Weight loss upon scrubbing was measured according to DIN 53778, after 10,000 cycles.

TABLE

Paint Sample No. 1 2 3 4

Brookfield Viscosity; initial

28 days 28,750 36000 35300 38900

37,450 37850 39600 45700

Gloss

60° 25.6 21.7 20.3 22.3

85° 46.9 43.6 41.7 43.2

Blocking Resist. Rating/% damage

24 hr/0.5 t/m² 4 0% 4 0% 4 0% 4 0%

24 hr/0.1 t/m² 4-5 0% 4-5 0% 4-5 0% 4-5 0%

Scrub Resistance,

7 days strokes 800/640 740/690 930/1000 700 740

Weight loss, g 0.506/ 0.456/ 0.388/ 0.405/

0.436 0.409 0.396 0.444

28 days strokes 1010/740 950/780 1050/1080 850/860 weight loss, g 0.42 0.306/ 0.390/ 0.370/

2/ 0.343 0.366 0.367

0.380

MFFT, °C 4

Similar results were obtained when Latex Examples 1 to 4 were blended with a styrene/2-ethylhexyl acrylate copolymer latex which was modified with methacrylic acid and sodium styrene sulfonate.

Latex Examples 1 to 4 can be formulated into a similar paint composition without being blended with a second latex. Such paint compositions will have excellent blocking and scrub resistance, but may be slightly more prone to cracking when applied to porous substrates at low temperature.

Example 5

To a suitable reactor is added an initial charge of 68.066 parts water, 0.806 part of a polystyrene latex, 0.75 part of itaconic acid and 0.01 part of a 40 percent solution of VERSENOL^* 120 brand chelating agent in water. The reactor contents were heated to 80°C. Three feed streams were begun at the same time (t=0). The first stream was an aqueous stream added over 360 minutes containing 9 parts water and 0.6 part sodium persulfate. The second stream was an aqueous stream added over 360 minutes containing 7 parts water and 2 parts of a 30 percent solution of Aerosol A-102 surfactant in water. The third stream contained 48.25 parts butyl acrylate and 20 parts styrene, and is added over 200 minutes. The temperature of the reactor contents was maintained at 80°C throughout the addition of these streams.

Beginning at time = 200 minutes, a fourth stream of 29 parts styrene and 0.5 part acrylic acid is added over a period of 100 minutes. A fifth stream containing 1.5 parts of an ethyleneureamethacrylate monomer in methyl methacrylate was added over a period of 50 minutes starting at time = 250 minutes. After all streams are added, the temperature of the reactor contents was increased to 90°C for 10 minutes. The latex was neutralized with potassium hydroxide. The product had a particle size of approximately 140 nm and contains 50.5 percent solids.

The T_g of the resulting polymer was evaluated by DMTA using the method described before. The T_g of the polymer prepared in the first stage (which includes the itaconic acid provided in the initial charge) was -2°C; that of the second stage polymer was 85°C.

Example 6

To a suitable reactor was added an initial charge of 64.421 parts water, 0.806 part of a polystyrene latex having a particle size of 30 nm, 0.5 part of itaconic acid and 0.01 part of a 40 percent solution of VERSENOL 120 brand chelating agent in water. The reactor contents were heated to 80°C. Three feed streams are begun at the same time (t=0). The first stream was an aqueous stream added over 360 minutes containing 12 parts water and 0.8 part sodium persulfate. The second stream was an aqueous stream added over 360 minutes containing 8 parts water and 2 parts of a 30 percent solution of Aerosol A-102 surfactant in water. The third stream contained 51 parts butyl acrylate and 19 parts styrene, and was added over 200 minutes. The temperature of the reactor contents was maintained at 80°C throughout the addition of these streams.

Beginning at time = 200 minutes, a fourth stream of 28.5 parts styrene and 1 part acrylic acid was added over a period of 100 minutes. An aqueous stream containing 0.2 part sodium persulfate was added over the course of an hour. An aqueous stream containing 3 parts water and 0.1 part ascorbic acid was added over a period of 20 minutes. The temperature of the reactor contents was then increased to 90°C for 10 minutes. The latex was neutralized with potassium hydroxide. The product had a particle size of approximately 140 nm and contained 50 percent solids.

The T_g of the resulting polymer was evaluated by DMTA using the method described before. The T_g of the polymer prepared in the first stage (which includes the itaconic acid provided in the initial charge) was -1 °C; that of the second stage polymer is 65°C.

The latex was then blended in a 90:10 weight ratio (solids basis) with a second latex containing styrene/n-butyl acrylate copolymer particles which were prepared in a single-stage polymerization. The resulting blended latex was then formulated into an 80 percent PVC coating having the following ingredients:

TABLE

Ingredient Parts by Weight Water 276.0

Natrosol 331 Plus' 3.0 Potassium Hydroxide (10%) 2.0 Orotan 731 (25%)² 10.0 Foamex 1488³ 2.0 Tiona 535⁴ 100.0 Durcal 2⁵ 350.0 Finntalc M15⁶ 90.0 Socal P2⁷ 45.0 Syloid ED50⁸ 20.0

Blended Latex (50% solids) 100.0 Acrylsol TT-935⁹ (30%) 2.0

Ηydrophobically modified hydroxyethyl cellulose ether, sold by Aqualon;

²A polyacrylic dispersant sold by Rohm & Haas;

³A mineral oil sold by Tego Chemie;

Titanium dioxide from SCM Chemicals;

⁵Calcium Carbonate from Omya;

Talc, sold by Finnminerals Oy;

⁷Precipitated Calcium Carbonate sold by Solvay; ^eA silica matting agent sold by Grace;

⁹An acrylic thickener sold by Rohm & Haas.

The initial viscosity, as measured on a Kreb Stormer viscometer at 25°C, was below 140 Krebs units. The MFFT is less than 0°C. The scrub resistance, as measured by DIN 53778 after 7 days drying, was 1300 strokes on each of two evaluations, with weight losses in the two evaluations being 2.428 g and 2.22 g, respectively. The resistance of the paint to cracking is evaluated by applying a 600 μm thick film of the coating on the non-printed side of plasterboard at 5°C, and drying. Only minimal cracking is seen.

A commercially available latex is also evaluated for scrub resistance in the same coating formulation. The scrub resistance is 700 and 1 100 strokes when tested twice, with weight losses of 1.93 g and 2.436 g, respectively. These stroke values are far lower than those of the coating of this invention, indicating a much poorer scrub resistance. In viscosity, MFFT and cracking, the commercially available latex performs similarly to that of the coating made from the blended latex of this example. Opacity, yellowness and whiteness are also similar for both coatings.

Example 7

To a suitable reactor was added an initial charge of 66.066 part water, 0.806 part of a polystyrene latex), 0.5 part of itaconic acid and 0.01 part of a 40 percent solution of VERSENOL 120 brand chelating agent in water. The reactor contents were heated to 80°C. Three feed streams were begun at the same time (t=0). The first stream was an aqueous stream added over 360 minutes containing 9 parts water and 0.6 part sodium persulfate. The second stream was an aqueous stream added over 360 minutes containing 8 parts water and 2 parts of a 30 percent solution of Aerosol A-102 surfactant in water. The third stream contains 56 parts butyl acrylate and 13.5 parts acrylonitrile, and was added over 200 minutes. The temperature of the reactor contents was maintained at 80°C throughout the addition of these streams.

Beginning at time = 200 minutes, a fourth stream of 29 parts styrene and 1 part acrylic acid was added over a period of 100 minutes. The temperature of the reactor contents was then increased to 90°C for 10 minutes. The latex was neutralized with potassium hydroxide. The product had a particle size of approximately 140 nm and contained 50.5 percent solids.

The T_g of the resulting polymer was evaluated by DMTA using the method described before. The T_g of the polymer prepared in the first stage (which includes the itaconic acid provided in the initial charge) was 3°C; that of the second stage polymer was 95°C.

Claims

CLAIMS:

1. An aqueous emulsion having an aqueous phase and a dispersed polymer phase, wherein the dispersed polymer phase comprises particles prepared in a multistage polymerization process in which a "first stage" monomer or mixture thereof is polymerized in a first stage, wherein the first stage monomer or monomer mixture polymerizes to form a polymer having a T_g of no greater than 25┬░C, and a "second stage" monomer or mixture thereof is polymerized in a second stage, wherein the second stage monomer or monomer mixture polymerizes to form a polymer having a T_gof greater than 60┬░C, the monomers polymerized in said first stage constituting from 50 to 95 percent of the weight of the multistaged particles, and the monomers polymerized in said second stage constituting from 3 to 50 weight percent of the weight of the multistaged particles, wherein the multistaged particles contain in polymerized form from 0.1 to 2 percent by weight (based on the multistaged particles) of an addition polymerizable, ethylenically unsaturated monomer containing at least two carboxyl or carboxylate groups and optionally from 0.5 to 3 percent by weight of an addition polymerizable ethylenically unsaturated monomer containing a single carboxyl or carboxylate group, provided that when the addition polymerizable, ethylenically unsaturated monomer containing a single carboxyl or carboxylate group is not present, the amount of the addition polymerizable ethylenically unsaturated monomer containing at least two carboxyl or carboxylate groups is from 0.85 to 2 percent by weight of the monomer content of the multistaged particles.

2. The aqueous emulsion of Claim 1 wherein said first stage monomer or mixture thereof includes a predominant amount of an alkyl acrylate wherein the alkyl group contains at least 4 carbon atoms.

3. The aqueous emulsion of Claim 1 or Claim 2 wherein said second stage monomer or mixture thereof includes a predominant amount of styrene, acrylonitrile, or a mixture thereof.

4. The aqueous emulsion of any one of the preceding claims wherein said addition polymerizable ethylenically unsaturated monomer containing at least two carboxyl or carboxylate groups forms part of the first stage monomer mixture.

5. The aqueous emulsion of any one of the preceding claims wherein said addition polymerizable ethylenically unsaturated monomer containing at least two carboxyl or carboxylate groups is itaconic acid, fumaric acid, succinic acid, maleic acid or a mixture of two or more thereof.

6. The aqueous emulsion of any one of the preceding claims wherein said addition polymerizable ethylenically unsaturated monomer having a single carboxyl or carboxylate group is acrylic acid or methacrylic acid.

7. The aqueous emulsion of any one of the preceding claims wherein said first stage monomer or mixture thereof constitutes from 60 to 80 weight percent of the monomer content of the multistaged particles, and the second stage monomer or mixture thereof constitutes from 40 to 20 weight percent of the monomer content of the multistaged particles.

8. An aqueous emulsion as claimed in any one of the preceding claims in which the first stage monomer or monomer mixture polymerizes to form a polymer having a T_g of no greater than 10┬░C.

9. A method of preparing an aqueous emulsion having an aqueous phase and a dispersed polymer phase, which method comprises preparing multistaged particles of the dispersed polymer phase by polymerizing a "first stage" monomer or monomer mixture in a first polymerization stage, wherein the first stage monomer or monomer mixture polymerizes to form a polymer having a T_g of no greater than 25┬░C, and thereafter polymerizing a "second stage" monomer or monomer mixture in a second stage, wherein the second stage monomer or monomer mixture polymerizes to form a polymer having a T_gof greater than 60┬░C, wherein the monomers polymerized in said first stage constitute from 50 to 95 percent of the weight of the multistaged particles, and wherein the monomers polymerized in said second stage constitute from 3 to 50 weight percent of the multistaged particles, and wherein the multistaged particles also contain in polymerized form from 0.1 to 2 percent by weight of an addition polymerizable, ethylenically unsaturated monomer containing at least two carboxyl or carboxylate groups and optionally from 0.5 to 3 percent by weight of an addition polymerizable ethylenically unsaturated monomer containing a single carboxyl or carboxylate group, provided that when the addition polymerizable, ethylenically unsaturated monomer containing a single carboxyl or carboxylate group is not present, the amount of the addition polymerizable ethylenically unsaturated monomer containing at least two carboxyl or carboxylate groups is from 0.85 to 2 percent by weight of the monomer content of the multistaged particles.

10. A blend comprising a mixture of an aqueous emulsion as claimed in any one of claims 1 to 8 with a second aqueous emulsion having dispersed polymer particles with a T_g of below 15┬░C, wherein the weight ratio of the aqueous emulsion of claim 1 to the second aqueous emulsion is from 85:15 to 98:2 on a solids basis.

11. A coating composition comprising an aqueous emulsion as claimed in any one of Claims 1 to 8, or a blend as claimed in claim 10.

12. A coating composition as claimed in Claim 11 which contains less than 8 percent by weight of an organic cosolvent and less than 8 percent by weight of a coalescent.

13. The coating composition of Claim 12 which contains essentially no organic cosolvent and essentially no coalescent.

14. A coating composition as claimed in any one of claims 11 to 13, which has a pigment volume concentration of from 30 to 80 percent.

15. A coating composition as claimed in any one of Claims 11 to 14, which forms a coating having a scrub resistance after 7 days drying of at least 640 strokes, determined as an average of two samples according to ASTM 2486.

16. A coating composition as claimed in any one of Claims 11 to 15, which exhibits a Byk blocking value after 4 days of no greater than 1.75 N/cm².