WO2001081481A1

WO2001081481A1 - Polymer modified inorganic coatings

Info

Publication number: WO2001081481A1
Application number: PCT/AU2001/000377
Authority: WO
Inventors: Robert Frederick Barber
Original assignee: AUSTRALIAN OXIDES Pty Ltd
Current assignee: AUSTRALIAN OXIDES Pty Ltd
Priority date: 2000-04-19
Filing date: 2001-04-04
Publication date: 2001-11-01
Anticipated expiration: 2002-10-19
Also published as: GB0223342D0; CA2405462A1; GB2378439B; AUPQ701300A0; NZ521791A; GB2378439A

Abstract

The invention relates to an anhydrous cementitious coating composition consisting of a hydraulic cement in the range 30-70 % w/w polymeric particles having a Tg less than 25 °C in the range 5-25 % w/w, and mineral extender particles in the range 5-50 % w/w. The cement to mineral extender particles are in the weight ratio 5:1 to 1:1. When mixed with potable water these compositions form useful decorative or protective coating compositions for application to a range of substrates.

Description

POLYMER MODIFIED INORGANIC COATINGS

This invention relates to inorganic coatings and in particular to polymer modified inorganic coatings. Background to the Invention Coatings have been used for many years to enhance the appearance and provide protection for substrates. For example, coating compositions are widely used as architectural and house paints. The substrates to which these coatings are applied include timber, unreinforced and steel reinforced concrete and bricks as well as previously coated substrates. Typically the coating compositions consist of a film forming binder in which are dispersed pigments or colorants that provide opacity and/or colour to the coating. The binders are generally either oil based where the binder is usually an alkyd resin or water based where the binder is a film forming latex. In more recent times latex based coatings have become the predominant type of coating for residential and commercial structures as they do not have high levels of organic solvent present. They are thus safer to use and less polluting to the environment.

With dimensionally stable substrates such as concrete or brick work it is quite common to apply a cement bound render coat. This render coat is usually of a composition similar to brick laying mortar and consists of sand, lime and Portland cement as well as optional colourants. This render coat is trowel applied after mixing the ingredients with water and normally a three coat system is used to provide a trowel applied coating having a total thickness of typically at least 10mm.

These cement containing, cementitious render coatings can crack from render coat shrinkage and from differences in expansion rates between the substrate and the render coating. This cracking can be exacerbated by poor adhesion between the render coating and the substrate. To alleviate this problem a bonding coating of a latex resin is sometimes applied to the substrate for the render coat. While this improves the longer term performance of the render coat it necessitates the additional time and expense of a further coating. Latex resin bound render coatings without cement have been proposed to replace the traditional cementitious render coat discussed above. These products offer the advantage of being able to be stored in a "ready to use" premixed form ensuring more accurate incorporation of colour and texturing aids. This leads to more consistent field performance. These latex based render coats have achieved wide- spread acceptance. However, they are relatively expensive. The binder in these coatings, is usually an acrylic latex. While these render coats are able to provide the desired decorative appearance to the substrate they also lack some of the long-term protective properties desired of such coatings.

One such protective coating characteristic is resistance to chloride ion penetration. This is an important property for steel reinforced concrete and particularly so when this is exposed to atmospheres containing salt. Salt induced corrosion of reinforcing steel is a particular problem with concrete structures in or close to maritime environments - specifically within 3Km of the coast. In Australia over 90% of structures are located within this distance from the coastline.

Polymer or latex modified cementitious (containing cement) trowel applied mortars have been proposed to improve the performance of such mortars. The mix proportions of most latex modified mortars are in the range of cement to fine aggregate (sand) ratio of 1:2 to 1:3 (by weight), polymer to cement ratio of 1:5 to 1:20 and water to cement ratio of 1:3 to 2:3.

Other workers have proposed the use of polymer latexes or powders in cementitious compositions.

US 5,806,609 (BASF) relates to adding alkyl phenol ethylene oxide surfactants to a cementitious composition to improve the adhesion of the composition to polystyrene foams. The cementitious compositions contain polymer powder but it is typically present at very low levels such as 3% in Example 1. US 5,753,036 (Air Products & Chemicals Inc) discloses mortar compositions that involve the use of acrylic latexes stabilised with partially hydrolysed poly(vinyl alcohol). The powdered polymer content is preferably in the range 1-20% and more preferably in the range 2-12%.

US 5,935,699 (Barber) discloses very small amounts of polymer being added to cementitious compositions. Other workers have used aqueous latexes but these do not lead to an anhydrous cementitious compositions. Summary of the Invention

This invention provides in one form an anhydrous cementitious coating composition comprising: hydraulic cement in the range 30-70% w/w polymeric particles having a Tg less than 25°C, preferably less than 15°C in the range 5-25% w/w, and more preferably in the range 8-20% w/w; mineral extender particles in the range 5-50% w/w and; wherein the cement to mineral extender particles are in the weight ratio 5:1 to 1:1.

Preferably the hydraulic cement percentage is 40-65% w/w and more preferably 50-60% w/w.

Preferably the polymeric particles are meth(acrylic) polymers. Preferably the mineral extender includes ceramic microspheres and more preferably the major proportion of the mineral extender consists of ceramic microspheres.

In an alternative form this invention provides anhydrous cementitious coating compositions as defined above wherein up to 100% of the polymeric particles are replaced with a natural polymer. The term natural polymer means that the polymer is a natural product or derived from a natural product. Examples are casein and cellulosic polymers. Detailed Description of the Invention

The anhydrous cementitious compositions of the present invention are described as anhydrous in that they are intended to be free flowing powders and require the addition of water before application and use. Minor amounts of water may be present, for example water of crystallisation associated with the mineral extenders. However, provided the minor amount of water does not adversely effect the storage stability or final properties of the composition such compositions are regarded as anhydrous. The coatings are described as inorganic in that a substantial amount of the binder is inorganic. However it will be appreciated that the coatings are mixed with significant amounts of organic polymer and such hybrid coatings are described as polymer modified inorganic coatings.

The compositions of the present invention are prepared for use by mixing the anhydrous blend of dry ingredients with potable water to form a slurry suitable for application. Typically the products are conveniently applied in a similar fashion to latex paints using either a brush, spray equipment or paint roller. They thus differ from conventional render compositions that are trowel applied. While the compositions of the present invention do require the addition of water for application they do not require the addition of other resin or latex components. The amount of water may thus be varied without the cement to latex proportions being changed . It will be appreciated that the amount of water added not only effects the application properties of the composition but also the chemical and physical properties of the final cured coating. The amount of water that is added to the composition is usually in the range of anhydrous composition to water of 1:0.2 to 1 :0.5 and more preferably 1 :0.2 to 1 :0.4.

An important characteristic of the polymeric particles of the present invention is their glass transition temperatures (Tg). The term glass transition temperature is well known in the art and generally defines the onset of long range molecular motion wherein the polymer preserves the outward appearance of a solid but becomes rubbery and then tacky with increasing temperature and undergoes plastic flow and elastic deformation. The theoretical Tg is often calculated in accordance with the Fox equation l/Tg=Wi Ti. However, Tg can be measured in accordance with the differential thermal analysis method set out in the Journal of Paint Technology, Volume 41, pages 167-168 (1969) or by testing of softening points or thermomechanical analysis (TMA). In practice we find in most cases that the Tg as determined by the various available methods is close to the theoretical Fox Tg. However, in some cases significant variations can occur and we prefer to use Tg as measured on actual particulars using a measurement such as TMA. The polymer particles suitable for the present invention are usually in the form referred to as redispersible polymer powders. These are manufactured by a two-step process. Initially polymer latices are prepared by emulsion polymerisation. Before these are spray dried the latexes are combined with bactericides, spray drying aids and antifoaming agents. Anti blocking agents such as ultrafine clays, silica and calcium carbonate are added to the polymer powders during or after spray drying to prevent caking of the powders during storage. This is necessary because the Tg of the powders is less than ambient temperature. The preferred polymeric particles are prepared from acrylic and methacrylic acid alkyl esters such as methyl methacrylate and butyl acrylate. However other monomers may be used and these include styrene, vinyl acetate and carboxylic acid containing monomers such as acrylic acid. Ethylene and butadiene are other possible monomers. Particularly suitable polymer particles are those supplied by Rohm and Haas under the trade name Drycryl DP- 2904. This is a 100% acrylic re-dispersible powder which has a Tg of 10°C and average particle size (before redispersion) of 45-75μm.

The compositions of the present invention are prepared as dry mixes using conventional mixing equipment. The slurried coating composition may be applied at appropriate rates to give dry film builds generally in the range 100-2000 μm although preferred dry film builds are in the range 400-1000μm. The compositions of the present invention when slurried with water usually have a pot life or useable consistency of an hour or more at ambient temperature and this allows sufficient time for application. Most of the water present in the slurried composition participate in reaction with the cementitious particles leading to films that do not exhibit as much shrinkage on drying as conventional latex paints or render coats. However, as the slurried compositions are applied as thin films relative to conventional render coatings, conventional concrete or mortar admixtures may be incorporated to reduce the amount of water needed for a given degree of liquid consistency and to reduce water evaporation. They also assist in consistent properties being attained under a range of application conditions. In some formulations that are applied onto porous substrates wicking of water from the slurried composition into the substrate may lead to a loss of water from the coating. In these applications anti-wicking agents may be used or alternatively a sealer coat may be applied to the substrate before applying the slurried coating. As the coating cures it has been found useful to apply a water spray to remove residual surfactants.

The mineral extenders are usually less than 100 μm in size and preferably less than 50μm and in preferred compositions include ceramic microspheres as a major component of the mineral extenders. Other suitable extenders include mica, clays, and diatomaceous earth. Other extenders that may be used include fly ash, blast furnace slag and condensed silica fume and the term mineral extenders also includes mineral particles that can participate in chemical reactions. These materials are commonly referred to as Pozzalanic materials. In this specification the term mineral extenders also includes inorganic pigments such as iron oxide, rutile and other opacifiers or colourants. In this invention the weight ratio of hydraulic cement to mineral extender particles is important and should be in the range 5:1 to 1:1. This ratio differs from many other cementitious compositions.

Minor amounts of organic colourants may also be used. However it will be necessary of course to select these so that they are stable in the relatively high pH environment of the compositions of the present invention. Other ingredients may be incorporated into the compositions of the present invention and these ingredients and their levels are those generally known and used in preparing aqueous coating compositions. Examples of such ingredients include anti-foam agents and rheology modifying agents such as water swellable or water soluble polymers. Surfactants or wetting agents may also be incorporated to aid the dispersion properties of the compositions and also assist their application properties. It will be appreciated that these additional ingredients are added in a substantially anhydrous form so that good storage life of the dry mix is maintained. The invention will be further described by reference to preferred compositions described in the following examples where all parts are expressed as parts by weight. Example 1

This example illustrates the preparation of an anhydrous composition: Acrylic polymer powder (Drycryl DP-2904ex Rohm and Haas) 16.000

Mica flakes (325 mesh) 1.450 Mica powder (53μm) 0.600

White Portland cement 58.000

Diatomaceous earth powder (5μm) 2.700

Casein natural resin powder 0.860

Anti-foam powder 1.900 Titanium Dioxide pigment 6.000

Ground granulated blast furnace slag powder 2.000

Methyl cellulose powder ('Methocel' H25) 0.030

"E-Spheres"SLG 9.000

Talc powder (30μm) 0.400 Flour (wheat flour) 0.560

Wetting agent (Naphthalene Formaldehyde

Condensate Resin) 0.500

100.000 The ingredients were combined by blending in an air mill. This composition consists of 58% hydraulic cement, 16% polymeric particles and 16% mineral extenders. The ratio of cement to mineral extender particles is 1:0.28. Example 2

This example illustrates the preparation of an aqueous composition from the dry mix of Example 1. dry mix of Example 1 1000 water 350

Water was added to the dry mix under gentle agitation. Agitation was continued for 10 minutes to achieve a uniform dispersion of the dry ingredients in the water. Example 3

This example illustrates the preparation and testing of the composition of Example 2.

The composition of Example 2 was applied in two coats at a nominal wet film thickness of 500μm per coat. A recoat interval of one hour was used. Sample Preparation

For the chloride ion diffusion test the test pieces were unglazed ceramic tiles and these were conditioned at 23±2°C and 50±20% relative humidity for 7 days prior to testing. For the adhesion strength test the test piece was a concrete block and after coating it was conditioned at 23±2°C and 60±10% relative humidity for 9 days prior to testing.

For the Taber abrasion resistance test the test pieces were unglazed ceramic tiles and after coating these were conditioned at 23±2°C and 60±10% relative humidity for 9 days.

Chloride Ion Diffusion Test

The chloride diffusion coefficient was determined at steady state condition using Ficks first law of diffusion to be 2.7 x 10^" m /sec.

There is no standard maximum value for chloride ion diffusion coefficient. Generally, specifications for chloride diffusion barrier coatings often quote a maximum value of D _Cι =5x10^" m /sec. The composition of the present invention as tested meets this criterion, and should act as an effective barrier against the ingress of chloride ions.

The degree of effectiveness in limiting chloride ion induced corrosion of reinforcement will be primarily dependent on the integrity of the coating, the specific salinity of the environment, the reinforcement cover and the required service life. Adhesion Strength Test

The mean adhesion strength of the coating composition of the coating, from five replicate measurements, was 1.9 MPa. These results are considered to be satisfactory since the mode of failure was cohesive failure of either the coating or the substrate concrete, indicating that the strength of the adhesive bond obtained exceeded the tensile strengths of the materials.

The AS 1580 test method is applicable to all generic types of film-forming coatings and substrates. There is no standard minimum value as this varies with the coating system, substrate and method of surface preparation. Generally, adhesion strengths for coatings or repairs on concrete substrates rarely exceed 2 - 2.5 MPa. Specifications for concrete repair materials typically require a minimum mean adhesion strength of 1.5 MPa between the repair material and the prepared substrate. The only reference to a recommended numerical adhesion strength value was found in the following British Standard:

BS 8204: Part 3: 1993 In-situ floorings - Part 3. Code of practice for polymer modified cementitious wearing surfaces This recommends a minimum value of 0.8 MPa for adhesion. The mean and individual test values exceeded the 0.8 MPa criterion. Abrasion Resistance Test

The average wear index of the coating composition was 101.3, as measured for 2 samples using the ASTM Taber abrasion test method. In isolation this value cannot be interpreted, but may provide some comparison with data for other materials tested using the same method.

The majority of available abrasion resistance test methods are only suitable for comparative assessment of materials, as the mechanism of abrasion does not usually model the mechanisms encountered in normal service environments. The Taber abrasion test method has relatively poor reproducibility. When numerical abrasion resistance values are to be compared they should be preferably be generated by a common laboratory.

The Taber abrasion test method is best suited to thin coatings, but is also used for ceramics and similar materials. It requires measurement of average weight loss over a number of wear cycles. The alternative Chaplin Method, which uses powered abrasive wheels for abrasion, is more aggressive and is best suited to tests of concrete substrates or relatively thick toppings on a comparative basis using the mean depth of wear.

Claims

Claims:

1. An anhydrous cementitious coating composition comprising: hydraulic cement in the range 30-70% w/w; polymeric particles having a Tg less than 25°C in the range 5-25% w/w; mineral extender particles in the range 5- 50% w/w and; wherein the cement to mineral extender particles are in the weight ratio 5:1 to 1:1.

2. An anhydrous cementitious coating composition as defined in claim 1 wherein the polymer particles have a Tg less than 15°C.

3. An anhydrous cementitious coating composition as defined in claim 1 or claim 2 wherein the polymeric particles are present in the range 8-20% w/w.

4. An anhydrous cementitious coating composition as defined in any one of claims 1 to 3 wherein the hydraulic cement is present in the range 40-65% w/w.

5. An anhydrous cementitious coating composition as defined in claim 4 wherein the hydraulic cement is present in the range 50-60% w/w.

6. An anhydrous cementitious coating composition as defined in any one of claims 1 to 5 wherein the polymer particles are meth(acrylic) polymers.

7. An anhydrous cementitious coating composition as defined in any one of claims 1 to 6 wherein the mineral extender includes ceramic microspheres.

8. An anhydrous cementitious coating composition as defined in claim 7 wherein the ceramic microspheres comprise the major portion of the mineral extender.

9. An anhydrous cementitious coating composition as defined in any one of claims 1 - 8 wherein the polymeric particles are replaced in part or totally with a natural polymer.