Waterborne Two-Component Isocyanate-Curing Polyacrylate Resins for Flooring Applications

Summary

Two-component isocyanate (2C NCO) cross-linked polyacrylate films offer unique properties such as excellent mechanical and chemical resistance and very good film formation. For water-based 2C NCO cure applications, special polyols and polyisocyanates are being developed. In this study, solvent-free secondary acrylic emulsions with different OH contents were prepared. These binders exhibit very good film formation and provide high barrier properties. Properties such as drying speed, hardness, chemical and mechanical resistance were examined. Against stains such as coffee, red wine or 48% ethanol, binders containing 2% OH show excellent barrier properties when cross-linked with a hydrophilic isocyanate cross-linker. For more demanding resistances such as MEK double rub or marker pen, hydroxyl content of up to 4% or 5% by weight is required. Although the properties of 2C NCO-cured acrylic resins are considerably better than 1C (single-component) acrylic resins, their use in floor applications is limited. This is primarily due to limited maximum film thickness and the lack of thermoplastic behavior in the acrylic copolymer. In this article, we demonstrate that hydroxyl-functional acrylic binders cross-linked with polyisocyanates improve the performance of water-based urethane coatings, combining the best properties of both technologies for use in floor applications.

Introduction

Two-component polyacrylate water-based binders cured with polyisocyanate (2C NCO) are a highly attractive alternative to solvent-based equivalents because they comply with regulations and do not require any labeling. However, there are two distinct disadvantages. First, in solvent-based coatings, the polymer and cross-linker are soluble in the continuous phase. In water-based coatings, however, the polymer and cross-linker must be emulsified in water and mixed after film formation. This situation can lead to serious problems, particularly with regard to the cross-linker. When hydrophobic cross-linkers are selected, high shear forces must be applied to provide good emulsification in order to prevent poor film properties. The hydrophobic cross-linker is often dissolved in auxiliary solvents before being added to the binder to improve compatibility. Alternatively, hydrophilically modified polyisocyanates can be used. Second, in solvent-based coatings the solvent is chosen inert to the cross-linker, while in water-based coatings the isocyanate group can easily be hydrolyzed. Although in general the rate constants for the reaction between aliphatic oligoisocyanates and water are lower than for primary alcohols, the excess of water relative to polymer-bound hydroxyl groups can cause significant hydrolysis. The disadvantage of hydrolyzed polyisocyanates is twofold. When an isocyanate group reacts with water, an unstable carbamic acid group forms that readily decomposes to form an amine group and carbon dioxide. The amine group will react with a second isocyanate group to form a urea bond (see Figure 1). Hydrolysis thus leads to cross-linking by sacrificing a second cross-linking group; therefore, water-based 2C NCO-based coatings are always formulated with an excess of isocyanate compared to free hydroxyl groups. The resulting carbon dioxide, particularly in thick films, such as coatings with dry film thickness greater than 50 μm, leads to blister formation that affects both the aesthetics and the integrity of the film. Secondly, although the reaction of the amine group with isocyanate also results in cross-linking, when one has previously been hydrolyzed, two isocyanate groups are required to achieve this. As a result, the efficiency of the cross-linker is significantly impaired.

Preparation of Binders, Formulations and Films

In this article, the properties of coatings based on solvent-free hydroxyl-functional (meth)acrylic emulsions with different OH contents will be discussed. A series of polymers with 2%, 3%, 4% and 5% (corresponding to 67, 100, 135 and 165 mg KOH/g, respectively) OH content via secondary emulsion polymerization were prepared. After dispersion of the polymer in water, the solvent was removed by vacuum distillation and a dispersion with 40% solids content containing less than 1000 ppm solvent was obtained. Due to real limitations encountered during polymerization in solution, the molecular weight of such binders typically ranges from 20–50 kD. The low molecular weight allows the polymer chains to have good flow during film formation. Thus, binders based on this technology exhibit very good gloss properties after drying. Furthermore, due to good film formation, the water barrier properties of these products are much better than a resin synthesized through conventional emulsion polymerization. Another factor enabling good barrier properties is the absence of (hydrophilic) surfactants. Surfactants required to stabilize the particles during emulsion polymerization remain in the film after drying and cause high water sensitivity of the film, and can even completely negatively affect film formation. To these secondary emulsions in the formulation, a hydrophilically modified NCO (Bayhydur® 3100) was added, the theoretical NCO:OH ratio was set to 1, and the solids content of the mixture was adjusted to 40%. This mixture was left to mix for 10 minutes and the test surface was applied 1 hour after mixing was completed. All films were applied at a wet film thickness of 100 μm using a wire rod. Before drying at high temperature (50°C) for 16 hours, the films were dried at room temperature for 4 hours to obtain approximately 40 μm dry film.

Drying, Cross-Linking and Film Formation

In this article, the film properties of different binders with OH content between 2% and 5% on polymer solids are discussed. Because NCO-functional cross-linkers plasticize the film during film formation, the drying time must be extended along with increasing NCO content (and thus increasing number of OH groups) to achieve the desired mechanical properties. As shown in previous studies, the drying times of water-based 2C NCO-cured binders can be controlled by selection of auxiliary solvents (cosolvents). For example, when solvents with high evaporation rates such as Dowanol PM are added, both surface drying and dust-free time can be reduced compared to a system applied without auxiliary solvent. When slow-evaporating solvents are used, solvent residue can be found in the coating film even after long drying times (days or even weeks), and drying times are significantly longer than for binders without solvent. In this study, to avoid the effects of auxiliary solvents on coating properties, all binders were formulated without auxiliary solvent.

Table 1. Drying time and König hardness of films as a function of binder OH content*

* All films were cross-linked with Bayhydur® 3100 at a 1 NCO:OH ratio.

Table 2. Storage modulus between cross-links (MC) and molecular weight of films containing binders with different hydroxyl numbers

Dust-free times for different OH numbers did not differ significantly and were all between 15 and 20 minutes. This is expected because dust-free time is thought to be related primarily to the evaporation rate of water, and the water content is similar for all tested binders. In contrast, surface drying (tack-free) time increased significantly with increasing OH number and the associated increasing isocyanate concentration. This is explained by the low viscosity/high amount of isocyanate required to cross-link these systems. The final hardness achieved at complete cure increased as a function of OH content due to increased cross-link density and increased concentration of urethane bonds in the cured films. Cross-linking levels of the films were confirmed by DMTA measurements. The storage modulus on the rubber plateau was measured to calculate the molecular weight between cross-links. As expected, higher hydroxyl contents in the binder result in higher storage moduli on the rubber plateau, and thus lower molecular weight between cross-links. The effect of increasing hydroxyl content from 4% to 5% appears to be limited, which indicates less efficient cross-linking and thus more side reactions. More information on cross-linking efficiency can be found in a separate publication.

Film Formation

Film formation was examined in more detail by atomic force microscopy (AFM). As can be seen in Figure 2, film formation for this type of binder is very good as expected, due to the limited molecular weight of acrylic polymers (MW<30 kD). Height differences on the Z axis are extremely small (<6 nm), indicating that truly smooth films are obtained. Furthermore, in the phase image, a homogeneous picture was observed without any traces of original particles. This shows that film formation was not hindered by vitrification due to cross-linking and that cross-linking occurred homogeneously throughout the films.

Figure 2. Film formation of acrylic copolymer with 5% OH content examined by AFM in height and phase contrast

In addition to AFM, electrochemical impedance spectroscopy (EIS) was used to gather information about the extent of film formation and barrier properties against water ingress. In the case of poor film formation (e.g., pinhole formation), the measured barrier against electric current would be poor, while for homogeneous undamaged films, significant impedance would be measured. As can be seen in Figure 3, the impedance does not change significantly with time, which means very good barrier properties and thus perfect homogeneous film.

Figure 3. Impedance as a function of exposure time to 0.1 M Na2SO4 solution of a cross-linked film containing a binder with 5% OH content, as determined by EIS

Changes in impedance upon water exposure can be attributed to water uptake by the film. This will naturally cause an increase in the conductivity of the low-conductivity polymeric film. By plotting the impedance as a function of time at high frequency (105 Hz), an indicator of water uptake can be obtained. Figure 4 shows that an increase in the number of OH in the acrylic binder results in a decrease in the equilibrium amount of water in the film. This can be explained by a higher degree of cross-linking. If cross-linking did not occur, an inverse trend would be expected because as the number of OH increases, there would be more hydrophilic groups in the polymer. Hydrophilic moieties cause higher water absorption. Furthermore, the differences between films with 4% and 5% OH content are relatively small; this shows that barrier properties do not develop further above certain cross-link densities. This situation is consistent with the DMTA results given in Table 2, which showed that cross-link density did not increase significantly when OH content was increased from 4% to 5%.

Figure 4. Water uptake profiles obtained by electrochemical impedance spectroscopy from binders with different OH contents

Resistances of Water-Based 2C NCO-Cured Binders

The good level of barrier properties of these binders against water ingress suggests that resistance to stains containing chemicals should also be good. Water, coffee, red wine and 48% ethanol in water stain resistance were found to be very good to excellent even for low-OH binders. Resistance to organic solvents was tested via MEK double rub. In this test, the number of rubs required to remove the coating from the surface is used as a measure. A clear trend was seen here: for binders with 4% OH and above, 1000 double rubs were reached without visible damage to the films. A similar trend was observed for marker pens of different brands. Higher OH content provided better resistance to stains caused by these marker pens. For the 2% binder, all colors were individually clearly detectable after cleaning with isopropyl alcohol. For binders containing 3% and 4% OH, all colors were visible but the intensity was significantly less compared to the 2% OH binder. In the binder containing 5% hydroxyl groups, only a faint shadow of the colors could be seen, with blue almost completely and black completely removed. This shows that very high cross-linking levels are required to pass this test. Black heel mark resistance was determined by observing the trace level as a standard heel with a high percentage of carbon black pigment was struck against a coated Leneta test card. If no black mark was found on the coating, a score of 5 was given; if there was an intense black mark, 0 was given. The test results for black heel mark resistance (BHMR) given in Table 3 are presented before and after the black mark is removed by wiping with a cloth. As expected, the film containing 2% OH binder showed poorer black heel mark resistance compared to higher OH numbers. In addition, it showed that the 3% OH-based film had a similar score to those containing 4% and 5% OH. This shows that 3% OH content is sufficient to obtain coating films that will not be damaged in the black heel mark resistance test.

2C NCO-Cured Binders for Floor Applications

Although the mechanical properties and chemical resistance of 2C NCO-cured acrylic resins are much better than 1C acrylic resins, their use in floor applications has so far been limited. One disadvantage of water-based 2C systems is the development of CO2 bubbles in thicker films (DFT>100–200 microns) due to hydrolysis side reactions. This risk of bubble formation naturally increases with increasing OH number. Furthermore, in high-wear environments (such as barber shops, busy shopping malls, garage floors, etc.), the mechanical properties of acrylate-based 2C NCO cure films are generally not adequate. The fundamental drawback of acrylic binders is that under high friction, the heat generated by friction can cause plastic deformation even in a highly cross-linked film. Mixing the acrylic polyol emulsion with an additional binder may be a solution to overcome these problems. In this article, we focus on using water-based polyurethane resins together with a 2C NCO-cured binder with 5% OH (NeoCryl® XK-555) in order to optimize the performance of the combined product. In the study conducted for this article, NeoRez® R-2180 and NeoRez® R-1010 were used as urethane emulsions. In addition, Bayhydur® XP 2700, which is a 65:35 mixture of a hydrophilically modified polyisocyanate and dipropylene glycol dimethyl ether (DMM), was used as the cross-linker in this study. Since both urethane emulsions are not hydroxyl-functional, the cross-linker can only react with NeoCryl® XK-555.

Figure 5. Overview of film properties of hydroxyl-functional water-based 2C NCO-cured binder (NeoCryl® XK-555) formulated with 20% by weight Bayhydur® XP 2700 (65:35 mixture of hydrophilically modified polyisocyanate and DMM) and a water-based polyurethane binder (NeoRez® R-2180). In particular, graded on a relative scale (0-5); 5 indicates excellent properties, 0 indicates poor performance

As shown in Figure 5, the urethane emulsion's resistance to thermoplastic deformation is greater than that of the acrylic 2C film. On the other hand, the resistances of the water-based 2C product are significantly better than those of the water-based urethane. Different mixing ratios were investigated to ensure optimal use of the individual properties of the 2C NCO-cured binder and polyurethane. Figure 6 shows that blending 30% by weight of the polyurethane product with acrylic emulsion containing 5% OH provides the ideal combination of film properties; all graded on a 0–5 scale, with 5 being excellent and 0 being poor. The thermoplastic behavior of the mixture improved significantly without compromising mechanical properties such as hardness or scratch resistance, or chemical resistance to paint, coffee or building chemicals.

Figure 6. Film properties of NeoCryl® XK-555 and NeoRez® R-2180 mixtures cross-linked with 20% by weight Bayhydur® XP 2700 (based on XK-555 solids)

In addition to high-wear applications, matte coatings are also an area where water-based 2C NCO-cured acrylic resins can make a significant contribution to improving performance. Matte surfaces are an aesthetic choice in floor coating applications to provide a natural look to wood substrates. One of the products that can be used to achieve these matte properties is NeoRez® R-1010, a water-based polyurethane emulsion. However, coatings based on this product do not have the hardness and scratch resistance required in floor applications. By blending this product with NeoCryl® XK-555 in a 75/25 ratio, an excellent balance between appearance and performance can be achieved. Mechanical resistances such as scratches and black heel marks improved significantly, and chemical resistance to household chemicals and disinfectants showed almost as strong an improvement as nearly pure 2C NCO film, as shown in Figure 7.

Figure 7. Film properties of pure polyurethane binder (NeoRez® R-1010) and 5% OH polyacrylate 2C binder (NeoCryl® XK-555) compared with the performance of a 75/25 blend of these products

Although the 2C NCO-cured polyacrylate film has a very high gloss level due to its formation properties, an ultra-matte appearance can be obtained in the mixture described above. By adding only a low amount of matting agent, the gloss level at 85°C can be reduced to below 10%. This shows that 2C NCO-cured acrylates can be converted to a very suitable coating material for parquet topcoats by using them together with non-hydroxyl-functional polyurethane binders.

Figure 8. Reduction of 85°C gloss level by adding matting agents for NeoRez® R-1010 – 75:25 blend with NeoCryl® XK-555. Formulation with 10% Easaqua XL-600/PC (80/20) by weight of binder

Conclusions

Water-based polyacrylate resins cured with 2C NCO can offer extremely good properties in terms of resistance (to stains and similar) due to high cross-link density. Because these products lack thermoplastic behavior due to high degree of cross-linking, they are not suitable for floor coating applications with high wear. When these products are blended with water-based polyurethane binders, strong resistance profiles can be combined with good mechanical properties to achieve excellent floor application properties. High-performance formulations can be created to achieve a solution suitable for both high-wear and aesthetic applications. Willem Jan Soer Senior Scientist, Acrylic Emulsions DSM Coating Resins B.V       Çiğdem Roman Product Manager, Paints, Inks, Adhesives IMCD Türkiye    

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