Epoxy Paints and Coatings for Concrete Surfaces

Solvent-Free Epoxy Paints and Coatings

Concrete surfaces in coating and protection applications employ solvent-free epoxy systems as outlined below. These are:

1. Solvent-Free Epoxy Paints:

Solvent-free epoxy paints are used when extreme chemical resistance or mechanical strength is required. These are protective coatings that deliver 200-250 microns of dry film thickness in a single coat. They are commonly used in concrete tank interior coatings and office facility coatings such as computer rooms. For vertical surfaces, thixotropic variants are also available.

2. Solvent-Free Epoxy Floor Coating:

Used on floors where solvent-free epoxy paints prove insufficient and higher mechanical strength is required. These are coatings applied in material storage areas and zones with rubber-wheeled forklift traffic. Depending on floor traffic, single-coat application with consumption of 800-1200 g/m² achieves 400-600 microns of dry film thickness.

3. Solvent-Free Epoxy Self-Levelling:

Self-levelling coatings are positioned between solvent-free epoxy floor coatings and highly-filled epoxy mortars. Self-levelling is applied where solvent-free epoxy paints do not meet practical requirements and/or higher mechanical strength is desired. Self-levelling is applied over primed concrete at layer thicknesses between 1-5 mm.

4. Solvent-Free Epoxy Mortars (Epoxy Grout):

Epoxy mortar is employed where self-levelling proves mechanically insufficient. Here the epoxy resin/filler ratio typically ranges between 1/8 and 1/9. Epoxy mortar is now a highly mechanically resistant epoxy grout, extending beyond paint and coating applications. Consequently, it can be applied at thicknesses of 10-50 mm. Beyond these, specialty solvent-free epoxy multi-layer coating systems, solvent-free epoxy laminated coating systems, solvent-free epoxy anti-static coating systems, water-based epoxy resin coating systems and solvent-free epoxy coaltar coating systems are available. The term "solvent-free paints and coatings" is defined as liquid coating systems applicable at film thicknesses ranging from approximately 200 microns to 50 mm. Flowability therefore serves as the criterion linking solvent-free paints, solvent-free floor coatings, self-levelling coatings and highly-filled epoxy mortars. Solvent-free paint coatings contain organic binder in higher proportions than the amount needed to wet fillers and create flow characteristics. Required binder content ranges between 25% and 40%. This ratio depends, among other factors, on the maximum particle size of the fillers. Increasing binder content improves flow properties, gloss and spread rate. However, compressive strength progressively decreases along with impact resistance. When high spread rate is required, it can be readily adjusted with increased binder content. Suitable mechanical spray equipment significantly increases the spread rate, facilitating application.

Additives and Auxiliary Materials in Solvent-Free Paints and Coatings:

These are additives provided to impart various properties to the solvent-free epoxy/hardener system.

Diluents:

Primarily supplied to reduce viscosity in solvent-free epoxy systems. They may have beneficial or adverse effects on film properties. Additionally, pine oil improves surface smoothness. Benzyl alcohol, furfuryl alcohol and nonyl phenol catalyze the reaction between epoxy resins and cycloaliphatic amines, and curing cannot be fully completed without these additives. Therefore, they are typically incorporated into the hardener. Epoxidised fatty acids increase material flexibility. Additionally, other diluents such as dibutyl phthalate and triphenyl phosphite are supplied to epoxy resin. Highly volatile, low-viscosity solvents exhibit the highest dissolving effect. However, when these types of solvents are used in high-build film systems, they do not evaporate as readily as thin-film solvent-based paints, potentially creating issues. Residual solvents in coatings may cause blistering and softening, particularly in sun-exposed areas and at elevated temperatures. Pigments colour the coating while fillers impart mechanical strength to the paint or coating. Inorganic compounds, typically metal oxides such as titanium dioxide (white), chromium oxide (green) and iron oxide (yellow, reddish brown, black) and their blends, are used as pigments.

Organic pigments may be used provided they meet colour stability requirements under light and chemical exposure and compatibility with alkaline hardeners.

Barite, talc, slate dust, silicates and glass wool are typical fillers. Carbonates should only be used where the system is not exposed to acidic conditions. Fillers may be spherical, rod-shaped or lamellar in structure. The coating's structural composition can be improved and pigmentation cost can be simultaneously reduced through appropriate pigment/filler combinations. With few exceptions, pigments and fillers can be added to both epoxy resin and hardener. Since the binder mixture comprises the larger portion, pigments and fillers are typically added to the epoxy resin component. Prior to premix preparation, compatibility and storage stability testing of fillers is recommended to prevent hard settling or viscosity increase that may arise from certain pigments such as zinc chromate, particularly in modified epoxy resins and hardeners. For the same reason, metallic pigments should not be mixed into hardeners. Pigmentation levels are limited by solvent-free binder viscosity. Typically, pigment-filler concentration ranges from 30-50% of total mixture. Three-roll mills, dissolver or similar equipment ensuring good pigment and filler dispersion and wetting are suitable. Hard fillers such as quartz dust and silicates should not be dispersed in three-roll mills as they damage the equipment. A dissolver is recommended for this purpose. Pigment pastes pre-dispersed in epoxy resin are suitable for pigmentation and these fillers. When calculating mix ratios, the epoxy resin content in pastes must be considered.

Moisture Content of Fillers:

Just as atmospheric moisture affects epoxy coating or paint surface formation, moisture content in fillers significantly impacts coating or mortar strength. Even 1% water content in a filler reduces the strength of an epoxy quartz mortar by 30% from baseline values achieved with dry quartz (moisture content <0.1%). Therefore, using dried filler is a fundamental rule for solvent-free epoxy resin formulations when achieving typical epoxy strength values is required. Moisture in fillers not only has a negative impact on mechanical strength but may also negatively affect surface quality, particularly in self-levelling coating systems. This can result in reduced flow properties and gloss as well as surface effects in the film.

Resin-Type Filler Additives:

These are defined as products that do not react with binder components. They are used to reduce paint and coating cost and improve certain technical properties. Hydrocarbon resins are the sole filler used in new formulations. Coal tar had been banned in various countries for toxicological reasons. Hydraulic engineering represents the primary application area for epoxy tar blends. High tar content up to approximately 40% significantly reduces reactivity; reduced reactivity can be managed by using fast-curing hardeners or adding accelerators.

Hydrocarbon Resins:

Used when necessary to maintain unlimited pigment usability in filler blends. Liquid, light-coloured hydrocarbon resins are suitable for this purpose. Approximately 75-80% binder to 20-25% hydrocarbon resin is recommended. Larger quantities may affect film properties, leading to migration and poor film curing. Adding auxiliary materials in recommended quantities affects film properties and mixture impact comparable to that achieved with coal tar addition. Hydrocarbon resins can generally be pre-mixed into the epoxy resin component and hardener without affecting storage stability. New formulations should in principle be tested for storage stability.

Surface Levellers:

Medium-viscosity binder blends, particularly when pigmented, tend to trap air in the film creating bubbles, pits or pinhole defects. At low temperatures, high atmospheric humidity and variable curing conditions, different surface textures may develop. Example additives provided to remedy these include TEGO Glide 1484 and BYK A500. Application at 0.3-0.6% by weight of binder helps prevent these effects. For orange peel appearance and increased surface hardness, Byk 301 is used at 0.1-0.2%. Byk P104 S has proven effective at approximately 0.5% by weight of binder against pigment separation in non-homogeneous pigment blends. Increased quartz content significantly raises coating compound viscosity. This also results in higher film thicknesses during application. Therefore, particular attention must be paid to air release from mortar formulations. Silicone oils have proven particularly effective additives. Pre-wetting the filler is recommended to facilitate better air release from the system, particularly for quartz dust where air adheres to the filler surface. This means the mortar compound contains no micro-air bubbles affecting technical properties and surface quality, or contains only minimal micro-bubbles.

Thixotropic Additives:

When solvent-free coatings are applied to vertical or inclined surfaces, thixotropic agents must be used to produce 200-300 μm thick films without sagging. Silica or silicate products and hydrogenated castor oil-based products are suitable for this purpose. These vary in form of application, method of supply and storage stability. Highly dispersed hydrophobic silica (fumed silica) is particularly suitable for prolonged thixotropic effect and is supplied at 2-3% by weight of total binder. Montmorillonite derivatives such as Bentone 27 and 34 can be used when dispersed in xylene or xylene/Antiterra U. Hydrogenated castor oil derivatives such as Thixcin R, Rheocin R and Rilanit Special can be used as 20% paste in ethanol or 20% solution in nonyl phenol at 1-2% by weight of binder. Fine dispersion of the additive within the binder is important for good thixotropic effect. Montmorillonite addition requires grinding in a three-roll mill for effective incorporation. The same applies to supply of 20% hydrogenated castor oil paste in ethanol to evaporate a large proportion of solvent. Silica-type thixotropic additives can be supplied by mixing in a dissolver.

Adhesion Promoters for Glazed Surfaces:

Paint adhesion to tile, glazed surfaces and glass is weaker than to other surfaces, particularly when exposed to water or alkaline environments. To improve adhesion, epoxysilanes for epoxy resin and aminosilanes for hardener at 3% by weight are suitable for both applied to binder.

Application - Concrete Surface Preparation

For a paint or coating application to fully meet job requirements, i.e., to protect the substrate against damage, adequate adhesion between the substrate and protective film layer must be ensured. Adequate surface preparation is a prerequisite for good coating adhesion. If the surface is poorly prepared, poor results are inevitable regardless of the binder system. Low-strength, weakened or cement-dusted thin concrete surfaces with loose cement mortar have low strength and remain weak for penetration. However, primed concrete primer cannot adequately spread on such closed surfaces. Therefore, such unsuitable concrete layers must absolutely be removed from the surface before application of a primed concrete primer recommended for all concrete coatings. Similar circumstances apply to dry, loose concrete surfaces described as "unsaturated concrete," which also have low surface strength. Patch concrete contaminated with oil, grease, chemicals and similar substances must be removed; solvent cleaning is often insufficient. The safest method is to scrape off and discard defective areas, then clean the concrete surface by grit blasting or flame cleaning without damaging the concrete. After surface preparation and cleaning, saturation of the surface with primed concrete primer is recommended. The objective here is to bind dust in the concrete and integrate capillaries and pores into the concrete, ensuring safer adhesion of the upper layer to the concrete. Primed concrete primer is absolutely necessary when concrete is very dense and compact, or when groundwater pressure is expected in the concrete's rear sections.

Mixing Process:

Before application, epoxy resin and hardener must be thoroughly mixed. Insufficient mixing will result in resin and hardener remaining separate or improper mix ratio, consequently preventing proper curing. The result will be tacky surface, weak mechanical strength and poor chemical resistance. Therefore, only uniform and homogeneous blends created by mixing the two components in the correct ratio will deliver the desired satisfactory film properties. Thorough mixing of primed concrete primers is evident from their optically homogeneous appearance. In pigmented blends, using different colour pigments for the two components will be beneficial in confirming complete mixing.

Application Methods:

Solvent-free epoxy paints and coatings are applied by hand or machine. Depending on paint or coating type and viscosity, application can be by roller, brush or squeegee. Single-component spray guns can be used for machine application of epoxy systems with medium to low viscosity having at least one hour pot life. Two-component spray guns are used for high-viscosity systems requiring pre-heating for spraying or systems with short pot life. Solvent-free epoxy paints can be applied at any desired film thickness. Thixotropic additives must be incorporated for application to vertical surfaces. Unlike solvent-based and emulsion paints, in some cases there is no requirement to apply several coats unless highly volatile solvents are added for thinning. However, as previously explained, it is advantageous for the initial coat on concrete surfaces to be a solvent-based primed concrete primer. A colourless, solvent-free primed primer can also be used to bind concrete dust and wet the concrete's lower surface. In multi-layer systems, depending on total film thickness, curing conditions relating to inter-coat adhesion must be considered. The typical time interval between two film applications is approximately 24 hours. Self-levelling mortars have higher binder content, making them considerably easier to apply than highly-filled epoxy mortars. Large spatulas or notched squeegees should be used. Coating capacity, depending on binder content, ranges from 15 to 30 m² per person per hour. This value can be significantly increased when using a suitable mechanical spray equipment such as plaster spray machines. These machines enable continuous application and can be used for systems with lower binder content. Spraying filled coatings to vertical surfaces with spray equipment eliminates the need for costly hand squeegee application. However, the filled coating is sprayed and then levelled with a squeegee.

Film Thickness:

The minimum applicable film thickness is approximately 1/3 of the maximum particle size of the filler blend. Consequently, the coating's protective function is unaffected by overhead loading, as particles do not create direct stress between the load and the substrate.

Curing Conditions and Cure Rate:

Temperature and atmospheric humidity exert strong influence on two-component paint curing and film surface properties. Solvent-free epoxy resin/hardener blends can cure at ambient temperature, low temperatures or elevated temperatures. Fast curing occurs at elevated temperatures, while at low temperatures the curing process is slower depending on hardener and epoxy resin reactivity. Due to application difficulties and delayed curing reaction, use of these hardeners below 5°C is not practical. To improve curing conditions in enclosed areas such as tanks, infrared radiators or hot air blowers capable of heating both the surrounding air and the surface can be used. This eliminates paint wetting difficulties during application and cooling difficulties caused by the applied surface. Standard hardeners, when cured in standard climate (23°C/50% relative humidity and 23°C/95% relative humidity), provide non-tacky surfaces within curing periods of less than one week. At low temperatures, depending on hardener reactivity and particularly when certain specialty hardeners are used, some delay occurs, but non-tacky curing does result. However, in some systems, a slight amount of residual tackiness may remain.

Pot Life:

Pot life begins immediately after epoxy resin is mixed with hardener and ends shortly before gelation. Pot life varies with the following factors: • Modified epoxy resins have somewhat longer pot life. • Varies with hardener type. • Changes with the addition of fillers extending pot life. • Varies with initial mixture temperature. High mixture temperatures reduce pot life. • Varies with mixture size. Reaction between epoxy resins and hardeners generates heat. The larger the mixture, the greater the heat output (exothermic). Therefore, mixture size must be suitable for the required working time. For this purpose, preparing mixtures in smaller batches is preferable. Additionally, distributing the generated heat, with cooling systems preventing temperature rise, are recommendations. An increase in viscosity or mixture temperature varies depending on binder reactivity and mixture size. At the end of pot life, application is no longer possible and film formation is insufficient. Low-viscosity cycloaliphatic polyamine hardeners gel after 15 minutes at 40°C. When working with "self-levelling mortars," the binder's reactivity is less critical because the mortar can be applied relatively simply and quickly. High-reactivity hardener coating systems that can cure very rapidly through exothermic heating in very compact masses must be spread immediately to the surface following mixing to prevent heat accumulation. Curing must occur at a specific surface temperature to prevent spreading problems.

Colour Change Under Water Exposure:

Under specific conditions, water exposure to cured paints and coatings can result in colour fading on the coating surface. This depends on curing conditions and the type of hardener used. This effect is more pronounced in aliphatic polyamine-based hardeners. Cycloaliphatic polyamines show less sensitivity to fading, but polyaminoamides are practically unaffected. Curing conditions are of vital importance. At approximately 20°C and relative humidity up to 95%, colour change is practically unobserved, but this effect becomes more severe at lower temperatures with increasing humidity. This effect is particularly critical when the temperature drops below the dew point, resulting in rapid cooling during curing. Due to hydration of the upper layer, a fine water film forms on the coating before curing completes. This reduces gloss and causes strong sensitivity when exposed to water even after prolonged curing. This effect occurs in both clear and pigmented coatings. Visual impression is more pronounced with bright-coloured pigments but less apparent with pale colours. Recommended measures to reduce water sensitivity after curing under critical conditions include using certain polyaminoamides in the hardener and simultaneously supplying silicone oil. The most effective silicone application rate has proven to be TEGO Glide B1484 at 0.2-0.6% by weight of binder. Heating enclosed tanks with infrared radiators or hot air blowers is advantageous throughout the entire curing period. Regardless of measures adopted, prior to coating application, curing conditions must be established to ensure satisfactory curing.

Inter-Coat Adhesion:

When temperature drops, high atmospheric humidity condensing on the not-yet-cured film alters the film surface through hydration, resulting in lower adhesion. The longer the coating remains under adverse curing conditions, the greater the effect. Here too, pre-testing under specified curing conditions is recommended for planning appropriate measures. When curing occurs at standard temperature, standard epoxy resin/hardener systems exhibit good inter-coat adhesion. Long intervals between successive coat applications should be avoided. Particularly if coatings will later be exposed to water, even a few days of open inter-coat spaces is disadvantageous. In such situations and after curing under adverse conditions, light surface abrasion can restore good adhesion.

Formulation Recommendations - Atmospheric Moisture Effect:

Binder surface exhibits variable water sensitivity shortly after short curing periods. This can result in a lighter or white colour. This can sometimes create practical application difficulties. A simple, highly effective laboratory test exists to examine formulation's optimal water performance. This is done by applying water droplets to the coating after curing and under various conditions. Droplets are removed after a short time and the extent of surface colour loss is graded.

Thermal Expansion:

The interaction of thermal expansion coefficient, film thickness, elastic modulus and maximum temperature difference determines the success of the applied protective coating. This is particularly important for outdoor coatings. The larger the self-levelling mortar's thermal expansion coefficient, the thinner the coating thickness. This is to prevent high stress in the adhesion zone between concrete surface and mortar. Under adverse conditions, stress forces may exceed the surface's tensile strength, resulting in substrate fracture due to good adhesion of epoxy systems.

Long-Term Performance:

Epoxy resin reactions at room temperature, depending on reactivity, are fundamentally completed within 1-7 days. During the weeks following application, strength increases continue to occur after curing. Over six months, compressive strength and particularly flexural strength increase to 110-120% of 7-day strength. Parallel to this post-cure strength increase, chemical resistance also increases, particularly capable of delivering notable improvement in critical environments. Results from open-air mortar tests have remained the same even 6-10 years after tests conducted under controlled climate conditions.

Weather Resistance:

Epoxy resin paints and coatings undergo adverse changes when exposed to outdoor conditions. Chalk formation and gloss loss are evident results. The interaction between UV radiation and moisture removes the binder's upper layer, exposing pigment particles leading to gloss loss and subsequently pigment loss. However, depending on total film thickness, surface damage is minimal, therefore no weakening of protective function is expected. Chalking can become visible after the first 3 or 6 months depending on weather. The cause of chalking and gloss loss is the aromatic basis of epoxy resins. Hardeners are secondary in importance. Yellowing can also occur, but yellowing only results from light exposure. The degree of yellowing is largely determined by hardeners. This effect appears slightly yellow with cycloaliphatic-type hardener while more pronounced yellowing is observed compared to aromatic-type hardener. Testing outdoor weather resistance is time-consuming under natural air conditions and "artificial weather conditions" represent a possible alternative. When comparing test results, the time dimension must also be considered.

Guideline Example Formulations:

The following formulations cover unpigmented and pigmented blends. These are formulations prepared for specific purposes and serve as examples of epoxy binder formulations with certain hardeners forming a basis for subsequent necessary developments.

Concrete Primer:

A low-viscosity concrete primer exhibits good spread power onto absorbent substrates such as concrete and similar surfaces. Primed concrete primer for concrete coatings serves functions of pore filling, strengthening the upper concrete layer to create sufficiently strong substrate for applied paints and coatings, and binding dust that causes adhesion weakness to concrete. For this purpose, formulations yielding film that cures on the surface in high-moisture environments with essentially no oily or surface adhesion should be used. When atmospheric humidity is high or moisture is present in the substrate during application and curing, use of moisture-sensitive hardeners can result in poor coating adhesion. This adverse effect results from hydration creating larger, more hygroscopic hardeners. Consequently, concrete primer is also affected to greater extent by moisture. In this problem, other factors such as binder system reaction speed, concrete quality and porosity also play roles. The popularity of low-viscosity solvent-free concrete primers stems particularly from the time-savings factor of not having to wait for solvent evaporation remaining in concrete. Solvent supply in these systems should not exceed 5%. However, it must be considered that solvent-free concrete primers have lower concrete spread capability compared to solvent-based products

Table 1: Example Solvent-Free Impregnation Concrete Primer Formula

Table 2: Example Solvent-Free Epoxy Floor Coating-White formula

Table 3: Example Solvent-Free Epoxy Self-Leveling-White formula

M. Namık Kayaalp - Chemical Engineer / Ecelak Paint Chemistry Ltd. References 1. Newton, D.S. (Ed.), Paint Technology Manual Part 4, Chapman and Hall, London 2. Paint Formulation, J.Boxal, Geoge Godwin Ltd.-London 1980. 3. SCHERING Industrie-Chemicalien, Bergkamen1 Germany, 1978 4. UPPC GmbH, Schemmerberger strasse 39, Baltringen, Germany, 1997 5. Huntsman Advanced Materials (Switzerland) GmbH Klybeckstrasse 200 P.O. Box 4002 Basel Switzerland 6. Epoxy Polymers, Edited by Jean-Pierre Pascault and Roberto J. J. Williams, WILEY-VLH Verlag umbH. 7. Paint and Coating Testing Manual Fifteenth Edition of the Gardner-Sward Handbook, Joseph V. Koleske,