Leveraging Surfactant Science to Design a Low-Slip Flow and Leveling Agent for Floor Coatings

Ingrid K. Meier, Darian Waugh and K. Michael Peck

Evonik Corporation, 7001 Hamilton Boulevard, Trexlertown, PA 18087

[email protected], (484) 954-6013

Abstract

High build 100% solids epoxy floor coatings are gaining popularity due to their high durability and low volatile organic compound (VOC) content.  However, these coatings require strong deaerators and oftentimes need additives to achieve good flow and leveling as well as defect control.  These challenges are worsened in high-gloss clear coats that cannot tolerate the presence of foam, craters, or haze.  While organo-modified silicone polyethers can be used to improve wetting, flow, and leveling in such systems, their high surface activity often negatively affects the performance of the deaerator and causes the resulting coatings to be unacceptably slippery.

This paper will discuss the chemical structural features needed to achieve wetting, flow, and leveling, and anti-cratering in organic and waterborne coatings. It will further describe how the principles of surfactant science were used to design a silicone polyether that can improve wetting, flow, and leveling without interfering with deaeration or contributing to haze or surface slip of the finished coating.

Introduction

Siloxanes are molecules that contain Si-O-Si linkages, and their structures can be tailored by varying the length and degree of branching of the siloxane backbone, as well as the number, position, and ratio of hydrophilic to hydrophobic groups present within the organo-modifications (Figure 1).  While organic functional groups are critical to the performance of organo-modified siloxanes, the siloxane backbone itself contributes to some of these molecules’ most unique characteristics including their high level of surface activity, enabling them to migrate to interfaces to lower surface tension within a time-frame consistent with their ability to move within the system.  The mobility of an organo-modified siloxane can be impacted by its size, molecular weight, and overall structure.^1-4  

Small organo-modified siloxanes are often used to lower the surface tension of aqueous, solvent-based, as well as high-solids and 100% solids, formulations.  They can be used to improve substrate wetting or compatibilize contaminants, thus preventing cratering and de-wetting.Higher molecular weight organo-modified siloxanes – typically between 1,000 – 20,000 g/mol – can function as surface control additives because,as one increases the length of the siloxane block within a polyether-modified siloxane, the interfacial activity of the molecule also increases; this can improve and/or impact properties such as flow and leveling, surface slip, and a variety of other surface properties. Due to the wide variety of coating formulation chemistries and properties, surface control additives have been designed for both specific applications and broader utility.  As mentioned in prior studies, there is not a “universal” surface control additive; therefore, many products are available and there is a need to understand them to take best advantage of their benefits.⁴

The decrease in a coating’s coefficient of friction (CoF) is widely referred to as the property of slip, which is characteristic of a smooth sliding motion across the coating; it may be beneficial or detrimental depending on the intended purpose of the coating.  The properties of the coating, such as its chemical composition and surface quality, as well as those of the object to be moved are important when measuring the surface slip, and these are reflected in both the static and dynamic coefficients of friction. Surface control additives with large polydimethylsiloxane (PDMS) segments and a high degree of surface activity can have a greater impact on the CoF, causing the surface to have a higher slip (lower CoF) and feel smoother and silkier.^4-6

The focus of this work was to study the effects of surface control additives on epoxy floor coatings where properties like wetting, flow, leveling, and anti-cratering are desired but increased slip cannot be tolerated.  A typical flooring application utilizes a liquid epoxy resin based on a diol like bisphenol-A or bisphenol-F, with an epoxy equivalent weight (EEW) in the range of 182 – 192.  It is also likely to contain a reactive diluent at 10-15 weight percent so that the overall resin base can maintain the necessary handling properties at ambient temperature.  An amine curing agent that has been chemically modified and/or formulated to provide specific properties to the coating is then added to initiate the formation of a high molecular weight polymer through the reaction between the epoxide and amine functionalities.  Formulators typically introduce the additives to the epoxy resin (Part A) side of the formulation while the amine (Part B) side is added with mixing at time of application.⁶           

In addition to the choice of epoxy resin and amine curing agent, it is important to understand the impact of various additives on the final floor coating properties.  CoF, clarity, color uniformity, control of foam, substrate wetting, inter-coat adhesion, and flow and leveling are extremely important and require a balance of additives to provide the required performance.  Whether to use a 100% solids solvent-free, solventbased, or waterborne epoxy formulation is a decision driven by environmental regulations, consumer demands, and performance requirements.^6,7

While one might think that 100% solids and waterborne epoxy formulations would have very few similarities and require completely different additives due to these differences, there are two types of waterborne epoxy curing types used depending on the EEW of the epoxy resin, one of which uses the same epoxy resins as the 100% solids systems.  This type of waterborne epoxy has a curing stage in which the water begins to evaporate after the waterborne amine begins to crosslink, and this process converts the system from a “water rich” to a “polymer rich” environment as the curing progresses.  The “polymer rich” phase that occurs prior to the full curing is chemically similar to the 100% solids epoxy system.^7,8  Often formulators introduce additives to the waterborne amine side as an alternative to formulating on the epoxy resin side; however, this may require different additives than used in 100% solids epoxy formulations.

This paper examines a series of organo-modified siloxane-based surface control additives to better understand their structure-property relationships and the effects that changes in additive chemistry have on the overall coating performance in 100% solids and waterborne epoxy floor coating formulations.

Results and Discussion

Experimental

The organo-modified siloxane surface control additives (SCAs) used in this study were either commercial products sold by Evonik Corporation or prototypes prepared in the laboratory using standard techniques.²

Two different epoxy formulations were prepared for this study.  The two-component (2K) 100% solids epoxy clear floor coating is shown in Table 2, and the waterborne pigmented epoxy coating is shown in Table 3.

The formulations in Tables 2 and 3 were prepared using a multi-step process in which the defoamer and surface control additive were added to Part A, which was mixed on a FlackTek™ speed mixer at 2,000 rpm for two minutes and allowed to equilibrate overnight.  Part B was prepared and then mixed with Part A on a FlackTek speed mixer at 2000 rpm for 2 minutes.
After the full formulation had been mixed, foam was intentionally incorporated by using an overhead mixer at 400 rpm for two minutes to mimic site mixing foam.  Coatings were applied using a drawdown bar to achieve a 5-mil wet film thickness on glass panels and Form P121-10N Leneta black scrub test panels (vinyl chloride/acetate copolymer), and 2 mm-thick castings were prepared by pouring the coatings into polycarbonate Petri dishes.

As it is difficult to photograph clear films, two new photographic methods were developed to properly show the visual differences between the 100% solids epoxy clear floor coatings.
The first method, which is best described as shadow imaging, uses a high intensity light shown through a clear coating drawn down on glass so that an image is projected onto a white background (Figure 2 – left).  As can be seen in the photograph of the projection (Figure 2 – right), the light refracts off surface defects to give “shadow” images that can be used to determine the surface quality of each clear coat.

A similar set-up was used to evaluate the foam and clarity of the clear coatings cast as
2-mm thick disks in polycarbonate dishes (Figure 3).  Unlike the camera set-up for shadow imaging, in this case a black background was used, and the high intensity light was shone at an angle perpendicular to the camera.  This configuration highlights the foam bubbles as well as the clarity of the coating.  Macrofoam is seen as large bright bubbles and microfoam is seen as smaller dots throughout the casting.

Dynamic CoF was assessed via a ForceBoard™ friction tester (Industrial Dynamics) equipped with a leather pad that was pulled across the surface of the coating drawn down on a glass panel as shown in Figure 4 (left).  The force to slide across the coating was measured for each coating, and then the values for all coatings were normalized using a 1 – 10 scale, where ten was the control formulation without a surface control additive. This enabled comparison of the impact that each additive had on the CoF of the coating.

            A method of sliding weights on a coating drawn down on a Leneta scrub chart mounted on glass was developed to measure static CoF as shown in Figure 4 (right).  The height that the coated panel needed to be raised before the weights began to slide was measured for each coating, and then the values were normalized using a scale of 1 – 10 with the control coating set to 10. This enabled a comparison between the test coatings and the control coating that contained no surface control additive.

Design of New Surface Control Additives

With the desired goal of designing an organo-modified polydimethylsiloxane surface control additive that could provide the required surface effects while not impacting the CoF, a study of structural variations was conducted.  As can be seen in Table 1, the structural features that were varied included the shape of the siloxane backbone, relative ratio of the siloxane backbone to the total molecular weight, size of the polydimethylsiloxane (PDMS) segments between organo-modifications, degree of organo-modification, hydrophilicity of organic pendant groups, and polarity of the end groups on the pendants.

Evaluations of Surface Control Additives in 100% solids 2K Epoxy Floor Coatings

To begin building an understanding of the structural features needed to deliver desired properties, the siloxanes listed in Table 1 were evaluated in the 100% solids 2K epoxy clear coat (Table 2).  Finding a structure that had the least impact on the CoF was of paramount importance for this floor coating; therefore, the relative static and dynamic CoF values were measured and are shown in Figure 5.  As might be expected, the four siloxanes with the highest degree of organo-modification – SCA#8, SCA#9, SCA#11, and SCA#12 – provide the highest CoF values and the least slip.

Other performance factors were also evaluated to identify an optimal SCA for this 2K 100% solids epoxy clear coat.  The compatibility of the additive in the formulation was assessed by looking for clarity of the cast films as seen in Figure 6 below.  Only the three additives with the largest PDMS segments – SCA#1, SCA#2, and SCA#3 – caused the coating to become hazy due to their incompatibility in the system. Additionally, several of the SCAs compatibilized the deaerator, causing it to lose its efficacy, as seen in the image of the coating containing SCA#4.

Figure 7 shows three shadow images of the clear coats in which the SCA’s ability to improve wetting, flow, and leveling or to cause cratering due to incompatibility can be seen. However, as can be seen in Figure 8, there is no correlation between the SCA’s compatibility with the formulation and its ability to reduce the effectiveness of the deaerator.  Additionally, as seen in Figure 9, the SCAs that cause craters in the final clear coat are not the same SCAs that cause haze.  While predictability of siloxane compatibility remains elusive, SCA#8 was found to have no negative impact on coating clarity, cratering, or defoamer/deaerator efficacy.

Figure 10 shows the ability of each SCA to improve the leveling of the coating when it was drawn down on either a glass panel or a Leneta scrub chart.  While all the SCAs significantly improve flow and leveling when the coatings are applied to the higher surface energy Leneta scrub charts, SCA#1 and SCA#8 provided perfect leveling.  Coatings applied to the lower surface energy glass substrates showed larger differences in leveling performance depending on the SCA used, and the best leveling was seen with the six SCAs that had low ratios of siloxane backbone to total polymer molecular weight – SCA#7, SCA#8, SCA#9. SCA#10, SCA#11, and SCA#12.

While it is difficult to perform systematic structure-property studies on organo-modified siloxanes due to the complexities of their chemical compositions, a comparison of two SCAs that differed only in the terminal end group on the pendant could be done.  Figure 11 shows that the only impact that substituting a hydroxyl end group (SCA#4) with an alkyl end group (SCA#5) had on the performance of the SCAs in this 2K 100% solids clear epoxy formulation was that the alkyl end cap-containing SCA had significantly less detrimental effect on the deaerator.  Still, as can be seen in Figure 8, hydroxyl-terminated pendants can be used to create SCAs that have no adverse effects on the deaerator if the overall structure of the organo-modified siloxane has been carefully designed.

Figure 12 compares the performance of six SCAs prepared using the same pendant groups but different siloxane backbones.  The ability of a given SCA to prevent craters or to negatively affect the deaerator can be impacted by differences in the siloxane backbone’s chain length and number of organo-modifications; however, a full chemical structure-property understanding remains elusive.  That said, SCA#8 provides the best all-around performance in this epoxy coating.

Evaluations of New Surface Control Additives in Waterborne Epoxy Floor Coatings

Evaluations of the SCAs in Table 1 in the pigmented waterborne epoxy formulation shown in Table 3 yielded interesting results.  In the waterborne epoxy, as in the 100% solids epoxy, SCA#8 had the least impact on both static and dynamic CoF values (Figure 13), and it performed best at improving the leveling of the coating when applied to both glass and Leneta scrub chart (Figure 14) while minimizing cratering fairly well (Figure 15).  The ability of SCA#8 to perform best in both epoxy formulations becomes less surprising when one considers that they are both based on the same epoxy resin and reactive diluent.  Therefore, once a significant amount of water has evaporated from the curing waterborne epoxy, the “polymer-rich” phase more closely resembles the 100% solids epoxy system.

   Unfortunately, the effects of organo-modified siloxane chemical structure on performance in this waterborne epoxy formulation could not be easily understood.  For example, highly “siliconic” SCA#1 significantly improved the leveling while much less “siliconic” SCA#4 and SCA#5 harmed the leveling and caused craters in this coating when it was applied to glass.

Due to the pigmentation, these waterborne epoxy coatings could not be imaged in the same manner as the 2K 100% solids clear coatings; therefore, any haze or negative impact of the SCAs on the deaerator was not able to be easily seen or quantitated.

Conclusions

Organo-modified siloxane surface control additives (SCAs) were evaluated in a 2K 100% solids clear epoxy floor coating to better understand the effects of their structural features on the properties of the cured films.  Siloxanes with the highest degree of organo-modification were found to provide the lowest slip – a critical requirement for use in floor coatings – and those with the largest PDMS segments caused the coating to become hazy due to their incompatibility in the system; however, no correlation could be found between the SCA’s compatibility in the formulation and its ability to reduce the effectiveness of the deaerator, and the SCAs that caused craters in the final clear coat were not the same SCAs that caused haze.  The tendency of these complex polymer distributions to associate with themselves and other system components may play a role in the phase separation and defoamer compatibilization behavior of the SCAs.

Ultimately, an optimized surface control additive – SCA#8 – was identified.  It provides excellent wetting, flow and leveling without negatively impacting the deaerator, causing haze or craters, or significantly increasing the slip of this 100% solids epoxy clear coat.

When the same SCAs were evaluated in a pigmented waterborne coating based on the same epoxy resin and reactive diluent as the 100% solids epoxy clear coat, the structure-property relationships became less clear, likely due to additional interactions involving the SCAs in the water-rich continuous phase. Surprisingly however, SCA#8 also provided the best overall performance in this waterborne epoxy floor coating.  This is understandable because, during the drying/curing process, the water evaporates leading the waterborne coating film to become more organic-rich and eventually chemically similar to the 100% solids system.

Acknowledgments

The authors would like to thank Michael J. Pauley and Jonathan E. Sefko for their technical contributions to this work.

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