Functional Additives

How to Avoid Bubbles in Moisture Cure Urethane Floor Coatings

Posted by Mickey Randall on Mar 13, 2024 3:09:00 PM

PUD Feature Image MCU

Moisture Cure Urethane (MCU) floor coatings are the best protection against dirt and contaminants. These coatings offer durable long-lasting protection and pleasing aesthetics. MCU coatings are applied on surfaces including wood and concrete floors for commercial buildings and retail establishments. The beauty and aesthetics of the high-quality moisture cure urethane coatings can sometimes be diminished by the presence of bubbles, which form during the curing of the coating. These bubbles likely will lead to a hazy or cloudy appearance in the finished coating and could lead to even more serious issues such as decreased surface protection and loss of adhesion.  


What Causes Bubbles in Moisture Cure Urethane Floor Coatings?

Urethane coating bubbles are caused by a variety of factors that can often be attributed to errors during preparation and application. This includes:

  • Over “working” the moisture-cured urethane during application 
  • Using incorrect roller covers 
  • Application to porous surfaces when they are rising in temperature 
  • Improper mixing  
  • Exposure to high levels of moisture 

The good news about application errors is that they can be remedied by closely following procedures and best practices. Let's discuss what that looks like.

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Preventing Bubbles in Finished Films

Our formulation chemist and product support groups have been working on ways to prevent Outgassing from occurring during the curing process and have identified the following 3 main areas critical to the avoidance of bubble formulation: 

  • Appropriate Applicators and Techniques 
  • Formulation Additives  
  • Proper Surface Preparation 

Let's begin by reviewing essential best practices to follow when applying MCU to a surface.

Related Article: Choosing a Low-VOC Polyurethane Dispersion


Appropriate Application Techniques 

When it comes to MCU being applied onto a surface for coating, it is important to select the correct applicator. MCU coatings are applied to a surface using a brush, lambs’ wool mop, or roller. Selecting the correct roller is important as some rollers are manufactured with a spongelike texture that increases the amount of air trapped in them and in the process of rolling on the MCU coating it introduces more air into the film, potentially leading to bubbles forming.  

Overworking is another issue we have discovered throughout our research as a cause of bubbles forming into a finished MCU Coating. Overworking an MCU Coating into a finished surface can cause bubbles in the final film, even when using the correct applicator.  

Applying the MCU Coating in the appropriate temperature environment is important as well.   When porous surfaces rise in temperature, the air in the pores heats up as well. The air expands and breaths out of the pores. Moisture-cured urethanes should be applied to porous surfaces when they are declining in temperature.

Mixing the urethane with a high-speed mixer will whip air into the product creating bubbles. 

Shaking containers will aerate the liquid and result in a film that contains bubbles. MCU Coatings cure by reacting with the moisture in the air or on the surface it is applied to. Carbon dioxide (CO2) is a by-product of the chemical cure process. In cases or locations where ambient temperatures and humidity are high, an excessive amount of CO2 can be generated and trapped in the film. This will create cosmetic defects like bubbles and low gloss.  Performance defects, such as loss of adhesion to the substrate may also be present.  


Formulation Additives to Limit Bubbling in Moisture Cure Urethanes

Two water-based urethane formulation techniques can be used to help limit bubbles: 

  1. Retarder Solvents 
  2. Bubble Breaking Agents 

The first technique is to add a Retarder Solvent to the MCU Coating. Bubbles are commonly entrapped in the film due to the early formulation of a dried “skin” layer. CO2 bubbles are a natural byproduct of the reaction, and should the film be present before the exhalation of these gasses, the bubbles will remain trapped within the finished coating.

A common solution has been the addition of a higher boiling point solvent, otherwise known as a Retarder Solvent. This solvent slows the drying process and prevents the “skin” layer from forming too soon, resulting in slower solvent evaporation while keeping the film permeable for longer. The CO2 can escape, leaving behind a smooth film when the curing is complete.  

You may also choose to add a bubble breaker solution to the MCU Coating per the additive suppliers’ instructions. This additive breaks the bubbles and prevents the entrapment of the CO2 in the tried coating. If the MCU Coating is applied by a roller, it may require a bubble breaker. As previously stated, some rollers trap air inside them and the air ends up in the coating when it is applied which can create bubbles.  

Be sure to use a mixer with a low RPM to prevent whipping air into the coating which will result in excessive bubbling.  

Related Article: What are my options with polyurethane coating formulations?


Proper Surface Preparation

MCU Coatings offer surface protection and are typically applied by coatings professionals. Cleaning and maintenance are quick and easy using a light clear and water or a microcloth cleaning head.  

To obtain a quality appearance and a long-lasting, fresh look; start with a surface fully prepared for the coating. Carefully reading through the application instructions and safety precautions are essential when working with MCU Coatings. The floor must be sanded and/or primed, as well as clean of debris and moisture. 

MCU Coatings are a labor-intensive undertaking but the results of a successful application are pleasing, both aesthetically and through their lasting performance protecting from dirt and other contaminants. If you need consultation or answers to anything related to Moisture Cure Urethanes (or anything related to polyurethane dispersions), leave a comment down below or press the Contact Us button. 

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Chase Corporation, Westwood, Ma. Founded in 1946 is a leading manufacturer of industrial products for high-reliability applications with a global customer base operating in diverse market sectors.  Chase specializes in the manufacture of waterborne polyurethane dispersions supplied to formulators for industrial, consumer coatings, and adhesives providing waterborne solutions.  Our outstanding manufacturing and technical support groups can provide your organization with a reliable global supply, unmatched quality, and superior technical support. 

Please contact us today to discuss your application.  

Topics: Polyurethane Dispersions, Moisture cure urethanes