Presented at the recent Polyurethanes 2011 Technical Conference hosted by the American Chemistry Council, this paper discusses the use of two-component ultra-low-VOC waterborne polyurethane sealers that can be used in conjunction with decorative concrete options to achieve both durability and aesthetic targets.

A recent trend, especially in sustainable building practices, is the use of a building’s existing concrete floor as a decorative element of the structure. Architects and designers are increasingly using this ubiquitous building component in more and more aesthetically pleasing ways. Diamond polishing, stamped concrete and decorative stains are being employed as durable yet beautiful floor options in both new construction as well as refurbishment work. However, these concrete surfaces and finishes can still lose their appeal should the wrong liquid accidentally come in contact with the surface since current sealers may not resist many common staining agents. The use of two-component ultra-low-VOC waterborne polyurethane sealers can be used in conjunction with decorative concrete options to achieve both durability and aesthetic targets.

Introduction

For the use of concrete as a decorative element in commercial buildings such as retail space, large grocery chains, office buildings, and light industrial structures, there are several types of concrete flooring options that may be used. First, an existing concrete floor may be ground and polished to a high gloss using diamond grinding equipment. Typically, the final polishing grit varies between 800-3000 grit. This method works well when the existing concrete floor is in good shape, has desirable aggregate content, size, and color, and is free of major cracks. After polishing, the floor may also be treated with a lithium silicate-based densifier, which creates a harder surface that resists micro abrasion and the subsequent concrete dust that can be generated. Additionally or separately, the concrete can be treated with a decorative stain, which imparts a desired color and look to its natural plain finish. By itself, this floor preparation method initially allows for a durable finish and good aesthetic qualities. However, over time the surface may be exposed to chemicals and foodstuffs, which can stain or even damage the concrete creating an uneven spotty look. Even with the use of a densifier, the concrete surface can remain porous enough for certain staining agents to penetrate and discolor it in places. In grocery stores, some common products can significantly alter the appearance of decorative or polished concrete if they are accidentally spilled on the floor. For instance, vinegar, pickles and relish will etch the concrete due to their acetic acid content and cause a discolored area, as well as a down-glossing of the affected area.

In order to address the staining or discoloring issue, applicators and owners have historically used acrylic-based sealers to attempt to protect their decorative concrete investment. These types of products are often generically referred to as “stainguard” or “guard” products. Many contained significant quantities of VOC and solvents which posed both odor and sustainability issues. More recent acrylic products, while reduced in VOC or solvent, often do not have the durability or chemical resistance needed for long tern performance.

Recent developments of waterborne polyurethane coatings and sealers address many of the desired attributes sought by the formulator, contractor, and owner such as:

- Chemical and Stain Resistance
- Ease of Use
- Low Odor
- Concrete Penetration
- Long Term Performance

By drawing on the proven power of polyurethane chemistry in many adjacent markets1, novel two-component waterborne polyurethane technologies have been developed that meet or exceed the desired targets of this growing market segment. For decades, solventborne polyurethane coatings have been considered the mainstay for high performance coatings used in architectural, industrial maintenance, corrosion, and construction applications due to their excellent mechanical and weathering properties. But increased governmental, regulatory, and sustainability pressures have created a need for coatings technology that would reduce or eliminate VOC, HAPS, heavy metals and/or other environmentally detrimental compounds. In the past decade, the first generation of waterborne polyurethane coatings was formulated and introduced to the market. While offering the chance to replace some of the VOC and solvents with water, many of these coatings still had in excess of 250 g/L of co-solvent. In addition, these coatings often fell significantly short of the solventborne polyurethane standard in chemical, abrasion and UV resistance. As with most emerging technologies, the learning curve was steep and the second generation of waterborne polyurethane coatings was developed with the goal of meeting or exceeding the desired traits of the solventborne polyurethane coatings but with significant reductions in VOC and solvent levels. These second generation waterborne polyurethane coatings have achieved the property goals and are “truly waterborne” – having 0-20g/L VOC levels. Due to the increased demand for higher durability concrete sealers, this waterborne polyurethane technology has been looked at as a replacement for the traditional acrylic stainguard products currently being used. Essentially, a “stainguard on steroids.”

Two-component waterborne polyurethane coatings

Experimental

Several waterborne resins were considered within the scope of this paper for use in the decorative concrete floor environment due to their desirable physical traits (Table 1). These traits include:

- Near zero-VOC formulas with excellent abrasion and chemical resistance
- No odor when applied in the field with other trades or building occupants present
- Adjustable gloss levels from high gloss to matte
- Ability to be used as a higher solids sealer for more porous decorative concrete or as a lower solids sealer for polished concrete and/or densified concrete.

Guide formulas were prepared using two Bayer waterborne resins along with the necessary additives (Table 2). The gloss levels were varied via the resins employed in the formula rather than the use of flattening aids. Bayhydur® XP 2547, a hydrophilic HDI trimer, was used as the hardener with a NCO:OH ratio of 3:1. This over-indexing can be accomplished with aliphatic isocyanates without detrimental bubble formation2. The guide formula coatings were applied at approximately 2-3 wet mils with a dry mil thickness of approximately 1-2 mils per coat. The purpose of the sealer is to penetrate the surface and should leave very little film build on the surface of the concrete. Dry times for the coatings ranged from 2 hours for the matte formulas to approximately 6 hours for the gloss formula. The trend was noted that as the formulas were made to be less glossy, i.e. more Bayhydrol A 2546 was used in the formula, the dry time was reduced. The Bayhydrol A 2546 resin is a faster curing resin than the Bayhydrol A 2542 resin.

A sample of concrete was polished to be used as a test substrate. The surface was diamond polished to a 1500 grit finish then divided off into horizontal sections using foam tape. The lines were divided vertically into 8 sections. These columns corresponded with overall resin solids levels, starting at 55% and decreasing in 5% increments to 20% resin solids. Each column was then tested using common staining agents such as those listed in Table 3.

Discussion

The testing revealed that the sealer prevented discoloration or gloss change of the concrete substrate when it was exposed to a variety of staining or corrosive substances commonly encountered in the decorative flooring market. Many of these staining agents would cause disbondment, discoloration, or milking of traditional acrylic sealers.4 The percentage of resin solids was investigated since there is such a wide range of porosity in the different types of decorative concrete surfaces – polished, stained, densified, or a combination of these treatments. For projects where the concrete has been polished to a lower grit level, for example 500-800 grit, the preference is to use a higher solids level which will fill micro pores and be more readily absorbed into the substrate. This is often the preferred method for acid stained concrete as well. For higher polished floors, 1500 grit or higher, or floors that have been densified, the standard guide formula can be modified to a lower solids content in order to help with penetration into the tighter concrete surface. This is achieved either by:

- Thinning a commercially available two-component waterborne polyurethane system with distilled water to the needed target solids content

- Purchasing a commercially prepared, lower solids two-component waterborne polyurethane system specifically formulated for tighter concrete applications

The preference of this author is for the second choice above for several reasons. First, the correct dilution rate is done by the manufacturer in a controlled environment which ensures the accurate and desired amount of resin is installed on the surface. Second, field dilution could possibly be done with water containing solids, iron, dissolved salts, or other contaminants that can affect the final properties or cure time of the system. Third and most important, a specifically formulated lower solids coating/sealer system will still contain the proper amount of the other necessary components such as flow and leveling aids, bubble breaker additives, surfactants, and other additives that are necessary to ensure an easy application, good wet out, and the desired aesthetically pleasing look, as well as the required final physical properties.

Conclusions

Polyurethane-based sealers offer a step up in stain resistance while protecting against corrosive agents such as organic acids. During application, this novel two-component waterborne polyurethane coating penetrates effectively, without objectionable odor. The sealer technology offers contractors and owners a long-term solution for protecting their decorative concrete investment while preserving the pleasing aesthetics of these ornate works of art.

Acknowledgements
The author extends his appreciation to the colleagues who were involved in the testing, data creation, and general consultation during the research and summary of these unique technologies: Kathy Allen, Jeanette Eastman, and Chris Sullivan.

References
1. Reinstadtler, Steven, “Intelligent Concrete Coating Solutions for Sustainable Construction”, CPI conference proceedings, 2009.
2. Wuehrer, Karl H.; Allen, Kathy, “Chemical Resistant Waterborne Polyurethane Topcoats for Flooring”, Journal of Protective Coatings & Linings, 2007 24(2), pg. 58-66.
3. K. Allen and P. Schmitt, “Greener Gambits in the Antigraffiti Game”, Journal of Architectural Coatings May 2009, pg. 38-47.
4. Interviews with professional flooring contractors in the decorative flooring industry, 2010-2011.

About the author
Steven Reinstadtler is the industrial marketing manager for Bayer MaterialScience’s Coatings, Adhesives and Specialties Division in Pittsburgh, Pa. He holds a BS degree in Chemistry with a Polymer Science option from the University of Pittsburgh. Steven joined the Polyurethanes Division of Bayer in 1987 as a chemist working on high performance prepolymers and spray elastomers. He transitioned into the Coatings Division in 2006, where he was responsible for the research activities of the Construction Coatings group as well as forward marketing activities. Steven is currently responsible for identifying new polyurethane coatings