Exterior Surface Concrete Paint Systems

Turkchem 26 Oct 2017 62 8 dk okuma

TURKCHEM

Introduction

The main substrates evaluated here are surfaces such as concrete and cement-based plaster. Both are normally cement-based with an inorganic binder. Most construction cements are hydraulic. That is, they set with water, and Portland Cement is the most widely used cement in this type of material. Portland Cement, a finely ground powder, sets and hardens when mixed with water because it contains mainly anhydrous calcium silicate. These and other cement inputs (in the form of hydration and hydrolysis) react chemically with water. The resulting structure can generally be seen as a colloidal gel with high internal surface area. Portland cement is typically prepared from limestone together with clay or clayey material. Raw materials are finely ground and heated to approximately 1500°C, where calcium carbonate decomposes to give calcium oxide. Partially fused clinker is ground to produce cement. Prior to this grinding, a quantity of calcium sulfate is generally added to adjust the setting time of the cement. Special cements are obtained by varying the proportions of raw materials and by mixing chemical additives during the clinker grinding phase. Portland cement forms the binding material in most construction concrete and plaster. It is used mixed with gravel (mineral aggregate) and sand, which regulate the hardness and durability properties of the final product. Sand and gravel also reduce shrinkage and cracking during the desired setting period. Modern concrete used in construction typically comprises: 1 part Portland cement 2 parts sand 4 parts gravel mixture. Cement-based plaster is applied as a thin layer to the surfaces of building structures to protect materials such as bricks and blocks against weathering. In some cases a cement-sand mixture is used, but in most cases slaked lime is also added. Typically, a cement plaster, to prevent shrinkage and cracking, comprises: 1 part cement, 2 parts slaked lime (hydrated lime), 6 parts sand mixture. There is no difficulty in painting dry, mature and sound surfaces. Conversely, some problems may be encountered when painting newly plastered surfaces or surfaces deteriorated by weathering. There are two main sources of painting problems on new cement surfaces. These are: 1) The presence of alkaline compounds in the cement itself, 2) Residual moisture from the water added for cement hydration. The combination of these two factors damages paints sensitive to alkaline effects. For this reason, paints used in such conditions must be of an alkali-resistant type. When complete drying is achieved, the risk of alkaline effect on cement surfaces is somewhat reduced against the danger of reactivation of salts in the underlying layer where moisture has not yet passed through. Other problems on such surfaces are variable and sometimes excessive porosity along with surface disintegration and cracks. Both conditions are generally found on surfaces that have remained in poor weather conditions for long periods. However, painting problems may also be seen on new, dry cement surfaces due to various pore formation throughout the surface.

Cement-Based Exterior Surface Paint Systems

Generally, although color and surface appearance are considered as primary factors when selecting paint, the characteristics of the substrate itself are also important. For this reason, the following properties of the substrate should be taken into account:

Moisture content,
Environmental conditions,
Alkalinity level,
Weather conditions,
Surface defects on the substrate.

Generally, paint formulations for cement surfaces are prepared to be resistant to high pH conditions, anticipating the presence of alkali salts. This can be accomplished either by using alkali-resistant polymers and pigments, or by applying an alkali-resistant primer (sealer) or primer before the topcoat is applied. Examples of alkali-resistant polymers include various vinyl resins and chlorinated rubber resins. These can be either solvent or water-soluble types (emulsion). Some alkali-resistant pigments include:

Titanium dioxide
Phthalocyanines
Iron oxides
Carbon black
Chromium oxide

The most important point in painting cement surfaces is the moisture content at the time of application. Excessive moisture generally adversely affects the adhesion of most paints to the surface, which reduces the performance of the applied paint. Even under good protection conditions, cement surfaces may require several months to cure. Generally, to achieve adequate drying, one week of dry air is required for every 5 mm thickness of wet structure.

New and Wet Exterior Concrete Surfaces

Since fresh and wet new concrete and plaster are difficult to paint, cement-based wet plaster paint is suitable for such surfaces and can be applied within a few days of the substrate forming. Cement paints are largely prepared with Portland cement, and special additives are given for application control, setting time and color. The adhesion property of such paints is the structural similarity between paint and substrate. A typical formulation showing approximate usage limits for white cement paint is shown in Table 1. As can be seen, cement paints are in powder form and are mixed with water to the desired consistency before use. Application is carried out with a brush in two coats with 16-24 hours between coats. Pigment usage level is quite critical and generally does not exceed 5%. Pigment usage above this ratio can result in reduced film integrity of the topcoat. However, pigment ratios of 5% and lower are sufficient to obtain a wide range of colors. When darker color tones are required, the use of gray Portland cement is appropriate. The types of pigments that can be used are limited to those with high color intensity due to low dosage rates and simultaneously high alkali resistance. Inorganic pigments are generally selected for this purpose. Other additives in the formulation modify application quality and film properties. By adding calcium or aluminum stearate, the paint application properties are improved and the water resistance of the dry paint film is also increased. Calcium chloride, which accelerates Portland cement hydration, is added to regulate the paint drying time. Slaked lime is sometimes added to reduce cracking tendency and as a filler in the cement paint.

New and Damp Exterior Concrete Surfaces

Other paint types can be safely applied to new cement surfaces that have completed approximately 4 weeks of curing. After this period, no moisture indication should be expected on normal surfaces. However, particularly in thickly applied areas, moisture content deep within the concrete may be observed, and therefore relatively porous paints and coatings may be selected in these areas. This ensures that moisture emerging during the long-term drying of the concrete does not damage the applied paints.

Emulsion Paints

The main paint groups suitable for application under these difficult conditions are prepared with emulsion polymers because they have greater permeability compared to similar paints prepared with solvent-based resins. The main components of an emulsion paint are pigmentation and binder; the binder here is a polymer dispersed in water. The pigments and fillers used are generally similar to those used in solvent-soluble paints. Pigments and fillers are dispersed in water, which functions as the volatile "carrier" component in the paint. Water contains small amounts of pigment dispersant additives that control the final viscosity of the paint as well as thickening additives. Typically, dispersants are surfactants such as sodium hexametaphosphate (approximately 0.5% by pigment weight). Thickening agents such as cellulose derivatives are in the range of 0.2-1.0% by weight of the paint. Or, less frequently, alkali-soluble polyacrylates are used for special application wall paints. Subsequently, pigment pastes are mixed with emulsion polymer and small amounts of additives necessary for paint stability and to facilitate film formation are added. Most emulsion paints contain additives such as foam suppressants, including special organosilicon compounds, fatty acid esters and fatty acid metal soaps, which are less than 0.1% by weight and replace foam formed during manufacturing. Biocides (or bactericides) and in some cases fungicides are also added. Biocides are added to control bacterial deterioration of paint during storage and fungicides are added to eliminate mold during paint service. The dosage levels of these compounds vary according to the type of material used; typically, biocides are added at a rate of less than 0.5% of paint weight, while fungicides can vary in rates up to 2% depending on the effectiveness of the compound and desired service life. Complex organic compounds have bactericidal or fungicidal effects in a wide range of emulsion paints, but most of these are specific. Examples of biocides used include chlorinated biphenyls, quaternary ammonium compounds and fluorine-containing sulfamides. Typical fungicides used are chlorinated phenols as well as dithiocarbamates and isothiazoline compounds. Emulsion polymer. The emulsion polymer used in paints must provide the necessary adhesion, durability, water resistance and chemical resistance properties to the paint. There are various types commonly used in wall paints: Examples include

Pure acrylic polymers,
Acrylics modified with styrene,
P.V.A. modified with acrylic,
Polyvinyl acetate (P.V.A.) modified with other auxiliary monomers such as versatic acid and ethylene.

Emulsion polymers differ from solvent-based polymers; these polymers are not in solution, but are in suspension form in water at small particle sizes of 0.3-3.0 μm. The film formation mechanism of these polymer emulsions and paints produced from them is complex and requires the passage of the water phase to the underlying layer and/or to the atmosphere. At the same time, polymer particles flow together or fuse with each other to enable film formation. However, effective fusion of emulsion particles, that is, film formation, does not occur below a certain critical temperature, which is a characteristic of the emulsion system. In practice, if this temperature is too high, a small amount of a high boiling point organic solvent, typically ethylene glycol monobutyl ether acetate, aromatic glycol ethers or complex ester alcohols or, less commonly, propylene glycol is added to the emulsion paint formulation to reduce it to assist film formation. Pigmentation. Emulsion paints formulated for application to incompletely dried walls are at a medium to high pigment volume concentration, but the accepted pigment volume concentration level must be consistent with adequate film strength. In such applications, 35-45% pigment volume concentration has been successfully used, and this percentage range allows the concrete surface to dry without film degradation. Typical formulations for quality exterior facade semi-matte white emulsion paints at approximately 36% and 42% P.V.C respectively are shown in Table 2 and Table 3. These paints are applied directly to the surface with a brush in two coats. Colors can be appropriately prepared using alkali-resistant pigments and/or pigment color pastes. Where dark colors are required, colors produced particularly from organic pigments are provided with a P.V.C. color base formula supplied with an appropriate filler combination where the white pigment is completely removed or minimized. M. Namık Kayaalp / Chemical Engineer / Ecelak Boya Kimya Ltd. Şti. Bibliography