Abstract:
This invention relates to a paint composition comprising a clay-containing pigment system, a latex binder, a polysaccharide thickener, and 0.005% to 2%, based upon the weight of the pigment, of a blocking agent which serves to prevent more than 20% of the polysaccharide thickener from being adsorbed onto the clay pigment surface, whereby the paint composition exhibits improved appearance and application performance.

Description:
FIELD OF THE INVENTION 
     The invention relates to the use of hydroxyethylcellulose-based thickeners in hydrophilic clay-containing emulsion paints. 
     BACKGROUND OF THE INVENTION 
     Prior to the present invention, latex paints containing clay as a pigment extender or containing calcium carbonate with clay impurities as the pigment extender, made it difficult to work with hydroxyethylcellulose (HEC) or hydrophobically modified hydroxyethylcellulose (HMHEC) from an application point of view. These paints often showed a gel structure and drew threads when applied with a roller. 
     In waterborne emulsion paints, cellulose derivatives like hydroxyethylcellulose (HEC), methylhydroxyethylcellulose (MHEC), methylhydroxypropylcellulose (MHPC), ethylhydroxyethylcellulose (EHEC) and carboxymethylcellulose (CMC) are used as rheology modifiers. These cellulose ethers thicken latex paints, providing the paints with stability, water retention and a pseudoplastic flow behavior. The cellulose ethers are used in an amount of 0.1% w/w up to 4.0% w/w on total paint, depending on the binder content of the paint. Hydroxyethylcellulose (HEC) and hydrophobically modified hydroxyethylcellulose (HMHEC) can be considered as the more popular types of cellulose ethers for use; clays are used as pigment extenders in emulsion paints in the United States while calcium carbonate is the most widely used type in European flat emulsion paint formulations, although in certain European areas, silicates and dolomites are used as well. 
     In paint systems containing hydrophilic clays, strong adsorption of HEC and HMHEC onto these clays occurs. This can affect the performance of the polymers in these paints dramatically. In distemper paints, which are often based on calcium carbonate containing clay impurities like montmorrillonite or bentonite, this results in a gelly paint structure. This gelly structure does not allow a good flow and application of the paint. For this reason, methylcellulose derivatives and carboxymethylcellulose are the preferred thickeners in these paints, since these cellulose ethers do not strongly absorb, so do not show the undesired gelation. Modification of HEC or HMHEC is needed in order to make them suitable for use as thickeners for clay containing flat emulsion paints. The above-described performance deficiencies of HEC and HMHEC as a result of their strong adsorption onto clay are also valid for ethylhydroxyethylcellulose (EHEC) and hydrophobically modified ethylhydroxyethylcellulose (HMEHEC). It is in meeting this need that the present invention provides an advance in the state of the art of the use of HEC, HMHEC, EHEC and HMEHEC as thickeners in emulsion paints containing hydrophilic clays or calcium carbonate extenders containing clay impurities. 
     SUMMARY OF THE INVENTION 
     This invention relates to a paint composition comprising a clay-containing pigment system, a latex binder, a polysaccharide thickener, and 0.005% to 2%, based upon the weight of the pigment, of a blocking agent which serves to prevent more than 20% of the polysaccharide thickener from being adsorbed onto the clay pigment surface, whereby the paint composition exhibits improved appearance and application performance. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     It has been discovered that the appearance and application properties of clay-containing emulsion paints, thickened with certain cellulose derivatives such as HEC, HMHEC, EHEC or HMEHEC, can be enormously improved by combining these cellulose derivatives with so-called pre-blocking agents. These blocking agents limit or even prevent the adsorption of the thickener onto clays and clay-containing extenders. 
     In accordance with this invention, a cellulose derivative thickening agent for clay-containing latex paints is selected from the group consisting of one or more of the following: 
     a. Hydroxyethylcellulose (HEC) with a molecular weight of from about 200,000 to about 1,500,000 and a hydroxyethyl molar substitution (HE-MS) of 1.5 to 4.0; 
     b. Hydrophobically-modified hydroxyethylcellulose (HMHEC) with a molecular weight of from about 50,000 to about 1,000,000 and a HE-MS of 1.5 to 4.0 with a hydrophobic modification chain of C 4  to C 22  alkyl or C 8  to C 20  alkylaryl in an amount of 0.01 to 2.0 percent by weight based on total weight of the hydrophobically-modified hydroxyethylcellulose; 
     c. Ethylhydroxyethylcellulose (EHEC) with a molecular weight of from about 200,000 to about 1,500,000 and a HE-MS of 0.5 to 3.0 and a degree of ethyl substitution of 0.3 to 1.2; and 
     d. Hydrophobically-modified ethylhydroxyethylcellulose (HMEHEC) with a molecular weight of from about 50,000 to about 1,000,000 and a HE-MS of 0.5 to 3.5 and a degree of ethyoxyl substitution of 0.3 to 1.2 with a hydrophobic modification chain of C 4  to C 22  alkyl or C 8  to C 20  alkylaryl in an amount of 0.01 to 2.0 percent by weight based on total weight of the hydrophobically-modified ethylhydroxyethylcellulose. 
     In accordance with this invention, the blocking agent is selected from the group consisting of one or more of the following: 
     a. Poly(ethylene oxide), poly(ethylene glycol), poly(propylene oxide) and poly(propylene glycol) with a molecular weight of from about 100 to about 1,000,000; 
     b. Poly(vinyl alcohol) with a molecular weight of from about 1,000 to about 500,000 and a degree of hydrolysis of from about 40 to about 90 percent; 
     c. Salts of poly(acrylic acid) and poly(methacrylic acid) and their acid equivalents with a molecular weight of from about 1,000 to about 1,000,000; 
     d. Nonionic surfactants of the type alkylarylethoxylate and alkylethoxylate with 4 to 100 oxyethylene (OE) units and a hydrophobic chain of C 8  to C 20  alkylaryl or C 4  to C 24  alkyl. 
     The upper limit of the blocking agent in the present invention is about 2.0% by weight, based on the total weight of the composition, with the preferred upper limit being 1.0% by weight. The lower limit of the blocking agent is 0.005% by weight, based on the total weight of the composition, with 0.1% by weight being the preferred lower limit. When this combination of thickener and blocking agent is used, the adsorption of the cellulose derivative onto clay or a clay-contaminated extender like Bianco Meudon Tipo A does not exceed 3 mg/g. Latex paints with the above-described thickener combinations do not have a gelly structure and can be applied to a substrate by roller without drawing undesired threads. 
     EXTENDER PIGMENTS 
     The types of extenders that can be used in this invention are basically two groups of products: a) clay-type extenders and b) calcium carbonate extenders contaminated with hydrophilic clay-types. 
     a. Clay-type Extenders 
     Examples of these clays are generally Kaolinites, chemically composed of SiO 2  (30-50 wt. %), Al 2  O 3  (30-50 wt. %), small amounts of alkalis (K 2  O, Na 2  O) and iron oxide (Fe 2  O 3 ). Other clays that may be used as extenders in this invention are calcined clays with low loss on ignition. Examples of such clays for use in the instant invention are: 
     
         ______________________________________                                 Loss on        Manu-    SiO.sub.2                          Al.sub.2 O.sub.3                                 IgnitionClays        facturer (wt. %)  (wt. %)                                 (wt. %)______________________________________Kaolin FP75  Dorfner  48       36     12Spesswhite   ECC*     47       38     13DB Plate 1000        Euroclay --       --     --China Clay Grade B        ECC      48       37     12.1China Clay Grade C        ECC      48       37     11.9China Clay Grade D        ECC      47.5     37     12.1China Clay Grade E        ECC      48.5     36.5   11.0Polestar 200 ECC      54.7     41.7   --Amazon 88    CADAM    45.1     37.8   13.9______________________________________ *ECC stands for English China Clay 
    
     b. Contaminated Carbonates 
     Calcium carbonates are available in relatively purified form like the Durcal-products, Omyacarb-series and Omyalite-series of OMYA. However, many producers of cheaper and less purified products also exist. These cheaper calcium carbonates can be contaminated with many types of clays. However, the contamination with very hydrophilic clays causes the problems with HEC in heavily pigmented paint systems. Examples of these clay contaminants are attapulgite clay, hectorite clay, montmorrillonites, and bentonite clay. One example of such a problem-causing calcium carbonate is the Bianco Meudon Tipo A used in the Italian paint market. 
     LATEX TYPES 
     The problem of strong structure build-up in the paint is mainly known in paint systems, where poly(vinyl acetate) VeoVA co-polymer binders are used as latex. 
     Examples of these binders which are partially stabilized with hydroxyethyl cellulose as protective colloid are: 
     
         ______________________________________Binder          Marketed By______________________________________Ravemul PC2     Enichem, ItalyVinavil 03V     Montedipe, ItalyUramul VV50     DSM Resins, The NetherlandsMowilith DM21   Hoechst, GermanyAcrilem 30WAC   ICAP, Italy______________________________________ 
    
     Latex paints of this invention provide excellent capacity to hide the substrate that it is to cover. The capacity to hide the substrate is controlled mainly by the pigment volume concentration (PVC) and the coating thickness. The PVC has certain limits dictated by economics and the capacity of the pigment binding vehicle. The latex paint of this invention, for example, when used in flat paints may have a PVC of from about 50 to about 100%. The coating thickness is determined in part by the viscosity developed in the composition. Other factors exist such as other coating properties and application characteristics, such as brush drag, flow, and leveling of the coating composition for developing the type of viscosity for a particular type of paint. 
     The practice of the invention is further illustrated by the following examples: 
     Adsorption experiments in the Examples were performed as follows: A 50 wt. % solids suspension of clay in water was prepared. 0.5 wt. % of thickener was added and dissolved in the pigment slurry or suspension and then stored for 24 hours at 25° C. The system was then centrifuged at 20,000 rpm for three hours. The amount of hydroxyethylcellulose or ethylhydroxyethylcellulose of the supernatant was determined via the Sealed Tube Zeisel method for measuring ethoxyl and hydroxyethyoxyl content. 
     These pigment slurries were also made in the presence of blocking agents in order to determine the effect of the blocking agent on the adsorption of the cellulosics onto clay and carbonate extenders. 
     
                       TABLE 1______________________________________Example 1The Adsorption of HEC and HMHEC Onto a Clay-ContainingExtender As a Function of Different Types of Pre-BlockingAgents                          Adsorption onto                          Bianco Meudon  Blocking                Tipo A of HEC.sup.aExample  Agent Type     % w/w    (mg/g)______________________________________1A     None           --       4.51B     Calgon N.sup.b 0.25     3.71C     Tamol 731.sup.c                 0.25     3.51D     PEG 400.sup.d  0.25     4.81E     Tamol 963.sup.e                 0.25     3.81F     Lutrol E 4000.sup.f                 0.25     1.91G     Lopon 885.sup.g                 0.25     4.01H     Pigment Verteiler A.sup.h                 0.25     0.01I     Airvol 803.sup.i                 0.25     0.01J     Sodium Dodecyl 0.25     11.2  Sulfate1K     Antarox CO430.sup.j                 0.25     4.01L     Antarox CO970.sup.k                 0.25     0.6______________________________________ .sup.a HEC: Natrosol ® 250 MBR, marketed by Aqualon Company. .sup.b Sodium hexametaphosphate wetting agent, marketed by Benckiser Knapsack. .sup.c Copolymer of diisobutylene maleic anhydride dispersant, marketed b Rohm &amp; Haas. .sup.d Poly(ethylene glycol) with molecular weight of 400. .sup.e Sodium salt of poly(acrylic acid), dispersant, marketed by Rohm &amp; Haas. .sup.f Poly(ethylene oxide), marketed by BASF. .sup.g Dispersant, sodium salt of poly(acrylic acid), marketed by Benckiser Knapsack. .sup.h Ammonium salt of poly(acrylic acid) dispersant, marketed by BASF. .sup.i Poly(vinyl alcohol), marketed by Air Products. .sup.j Nonylphenol ethoxylate (4 EOs), marketed by GAF. .sup.k Nonylphenol ethoxylate (50 EOs), marketed by GAF. 
    
     
                       TABLE 2______________________________________                         Adsorption onto                         Bianco Meudon Tipo  Blocking               A of HMHEC.sup.aExample  Agent Type    % w/w    (mg/g)______________________________________1M     None          --       6.71N     Calgon N      0.25     9.41O     Tamol 731     0.25     12.51P     PEG 400       0.25     4.01Q     Tamol 963     0.25     4.01R     Lutrol E 4000 0.25     0.01S     Lopon 885     0.25     4.11T     Pigment Verteiler A                0.25     0.81U     Airvol 803    0.25     0.01V     Sodium Dodecyl                0.25     11.7  Sulfate1W     Antarox CO430 0.25     5.81X     Antarox CO970 0.25     2.5______________________________________ .sup.a HMHEC: Natrosol Plus ® 330, marketed by Aqualon Company 
    
     The above-given examples 1A through 1X demonstrate that only specific products can be used as a blocking agent to limit the adsorption of HMHEC onto Bianco meudon Tipo A. Lutrol E 4000 (as marketed by BASF), a poly(ethylene glycol) with a molecular weight of approximately 4000, pigment Verteiler A (as marketed by BASF), an ammonium salt of poly(acrylic acid), Airvol 803 as marketed by Air Products), a poly(vinyl alcohol) (molecular weight about 10,000; 88-89% hydrolysis) and Antarox CO970 (as marketed by GAF), a nonylphenol ethoxylate (50 EO units) are able to reduce the adsorption of HMHEC onto the extender. These blocking agents are able to form hydrogen bonds with clay or the clay present as contaminant in calcium carbonate extenders. Higher molecular weight poly(ethylene glycol) (Lutrol E 4000) is more efficient than lower molecular weight material (PEG 400). Nonylphenol ethoyxylate surfactants become more efficient as a blocking agent when increasing the number of EO-units in the product (see Antarox CO970 versus Antarox CO430). The ammonium salt of poly (acrylic acid) is more efficient as a blocking agent than the sodium salt (see Pigment Verteiler A versus Lopon 885 and Tamol 963). 
     EXAMPLE 2 
     The differences in adsorption values are reflected in differences in paint properties of the paints based on the Bianco Meudon Tipo A and the various combinations of HMHEC and blocking agents. The following examples (2A-2T) exhibit the practice of the invention in a flat emulsion paint having a pigment volume concentration (PVC) of 94%. The paint samples were prepared under Cowles conditions with a high speed dissolver. The paint formulation is given in Table 3. 
     
                       TABLE 3______________________________________Paint Formulation Containing Bianco Meudon asPigment Extender; PVA: 94%; Solids: 58% w/w         Parts by Weight______________________________________Water           287.1Preservative CA24           1.0Pigment Verteiler A           1.0Calgon N        1.5Agitan 280      1.0Ammonia         1.0Bianco Meudon   665.0Texanol         2.0Ravemul PC2     33.0Thickener       by demandBlocking Agent  3.0Water           97.5Total           1093.1______________________________________ 
    
     The paint was thickened with Natrosol Plus® grade 330 HMHEC and Natrosol® 250 MBR HEC combined with different blocking agents. The test results are shown in Table 4 and Table 5. 
     
                                           TABLE 4__________________________________________________________________________Combination of (Natrosol ® 250 MBR) HEC With Different PreblockingAgents in a Flat Emulsion Paint (PVC 94) Based onRavemul PC2 With Bianco Meudon Tipo A As An ExtenderHEC  Blocking Agent            RollerExample(Wt. %)     Type        SV.sup.1                    SYV.sup.2                        Appearance.sup.3                               Application.sup.4__________________________________________________________________________2A   0.36 None        109                    800 --     --2B   0.28 Pigment Verteiler A                 100                    250 +      +2C   0.30 Calgon N    116                    1600                        --     --2D   0.24 Sodium Dodecyl Sulfate                 101                    900 --     --2E   0.31 Tamol 731   107                    700 --     --2F   0.33 Antarox CO970                 112                    250 +      +2G   0.34 Antarox CO430                 106                    450 +--    +2H   0.26 Airvol 803  110                    250 +      +2I   0.30 Lutrol E 4000                 114                    200 +      +2J   0.37 PEG 400     100                    500 +--    +__________________________________________________________________________ .sup.1 Stormer viscosity. .sup.2 Stormer Yield Value; maximum load in grams before spindle starts t rotate. .sup.3 Appearance of the paint with respect to gelly structure; ranked from strong gelation (--) to nongelly (+). .sup.4 Applicability by roller in terms of ease of spreading and drawing of threads; ranked from very poor (--) to good (+). 
    
     
                                           TABLE 5__________________________________________________________________________Combination of HMHEC (Natrosol Plus ® 330) With Different blockingAgents in a Flat Emulsion Paint (PVC 94) Based onRavemul PC2 With Bianco Meudon Tipo A As An ExtenderHEC  Blocking Agent            RollerExample(Wt. %)     Type        SV.sup.1                    SYV.sup.2                        Appearance.sup.3                               Application.sup.4__________________________________________________________________________2K   0.37 None        109                    650 --     --2L   0.40 Pigment Verteiler A                 108                    200 +      +2M   0.26 Calgon N    114                    1800                        --     --2N   0.26 Sodium Dodecyl Sulfate                 112                    600 --     --2O   0.36 Tamol 731   107                    550 --     --2P   0.27 Antarox CO970                 108                    150 +      +2Q   0.28 Antarox CO430                 106                    350 +--    +--2R   0.37 Airvol 803  109                    200 +      +2S   0.35 Lutrol E 4000                 114                    100 +      +2T   0.36 PEG 400     100                    300 +--    +__________________________________________________________________________ .sup.1 Stormer viscosity. .sup.2 Stormer Yield Value; maximum load in grams before spindle starts t rotate. .sup.3 Appearance of the paint with respect to gelly structure; ranked from strong gelation (--) to nongelly (+). .sup.4 Applicability by roller in term of ease of spreading and drawing o threads; ranked from very poor (--) to good (+). 
    
     EXAMPLE 3 
     Another experiment was performed to demonstrate that the effect of combining the cellulose ether with a blocking agent is also valid for EHEC and HMEHEC. EHEC was Bermocol E 351 FQ polymer and the EHEC was Bermocol EHM100 polymer. The influence of using a combination of HMEHEC and a blocking agent on the paint properties is shown in the flat emulsion paint (PVC94) based on Ravemul PC2 material. Examples 3A-3D illustrate these benefits (see Table 6). 
     
                                           TABLE 6__________________________________________________________________________Combination of (HM)EHEC with Lutrol 4000In a Flat Emulsion Paint (PVC 94) Based OnRavemul PC2 With Bianco Meudon Tipo A as an Extender    HME  LutrolEHEC    HEC  E 4000              SV.sup.1                  SYV.sup.2  RollerExample(wt %)    (wt. %)         (wt. %)              (KU)                  (g) Appearance.sup.3                             Application.sup.4__________________________________________________________________________3A   0.35    --   --   106 700 --     --3B   0.31    --   0.25 109 200 +      +3C   --  0.34 --   110 600 --     --3D   --  0.29 0.25 111 150 +      +__________________________________________________________________________ .sup.1 Stormer viscosity. .sup.2 Stormer Yield Value; maximum load in grams before spindle rotates. .sup.3 Appearance of the paint; gelly (--) to nongelly (+). .sup.4 Applicability by roller; good (+) to very poor (--). 
    
     EXAMPLE 4 
     In paint systems with strong hydrophilic clays like Spesswhite, the amount of blocking agent needs to be increased in order to limit the adsorption of HEC, EHEC, HMHEC and HMEHEC onto these types of clay. This is illustrated by the results of adsorption experiments shown in Examples 4A and 4B (see Table 7) using Lutrol E 4000 as a blocking agent. 
     
                       TABLE 7______________________________________Influence of the Addition of a Blocking Agenton the Adsorption of HEC (Natrosol 250 MBR)Onto Spesswhite, a Hydrophilic Clay ex ECC.                        Adsorption of HEC                        Onto SpesswhiteExample  Wt %      Blocking Agent                        (mg/g)______________________________________4A     --        --          10.04B     0.25      Lutrol E 4000                        9.84C     0.5       Lutrol E. 4000                        3.64D     1.0       Lutrol E 4000                        0.3______________________________________ 
    
     The combinations of HEC, EHEC, HMHEC or HMEHEC with blocking agents can be used in the form of a powder blend, fluidized polymer system or as a solution of subject materials. Products like poly(vinylpyrrolidone) and poly(acrylamide) are expected to work as a blocking agent as well.