New Acrylic Dispersion Technology for Woodwork and Decoration Applications

Introduction

Worldwide, initiatives are being undertaken to reduce the environmental impact of coatings by reducing VOC (volatile organic compounds) and TVOC (total volatile organic compounds). As a result, there is significant pressure towards acrylic dispersion technology. While this technology offers good performance, the market still seeks improvements in early water resistance, chemical resistance and dirt pickup. The versatility of binder systems enables reduced inventory levels and less complex supply chains. This article will discuss coating formulations and application results based on SETAQUA® 6799, the first product of this type launched on the market.

Morphology Control and Particle Stabilization in Emulsion Polymerization

Given the above considerations, the coating industry's objective is to reduce cosolvent demand for water-based coatings, with the ultimate goal of "zero VOC". Here we face the most important dilemma: how to ensure adequate film formation together with good curing at low temperatures, in other words, how to separate the polymer's glass transition temperature (Tg) from the minimum film formation temperature (MFFT). In acrylic dispersions with homogeneous morphology, the MFFT of the dispersion is always closely related to the Tg of the polymer. Early approaches to separating Tg from MFFT manifested themselves with the introduction of so-called core-shell dispersions in the 1980s. Core-shell structured dispersions are prepared using a sequential emulsion polymerization technique. Core-shell polymerization offers a route to facilitate film formation through, for example, softer shells surrounding a harder core material. The core polymer provides durability, gloss and low dirt pickup properties. The softer shell material, which can form film at a lower temperature, offers the opportunity for lower cosolvent levels in the paint. An alternative to the core-shell approach is the use of polymer dispersions with a feature called gradient morphology. During the synthesis of these dispersions, the polymer composition changes gradually during the polymerization process. Surfactants are a vital component in emulsion polymerization to achieve stabilization of polymer particles both during and after polymerization. Although some grafting may occur during polymerization, surfactants are physically attached to the surface of polymer particles in large measure. Desorption of surfactant during storage or film formation can cause particle instability or deterioration of coating properties. Surfactant also tends to leach from the coating film, as reported by Hellgren and colleagues, and will thus affect the final properties of the paint film. Solutions have been proposed to overcome these limitations on the use of conventional surfactants. The first of these is the use of reactive surfactants; surfactant molecules that, in addition to the amphiphilic portions of a conventional surfactant type, have a reactive group that can participate in free radical polymerization. Another way to avoid the use of low molecular weight surfactants is to use water-soluble polymeric or oligomeric stabilizers in the emulsion polymerization process. Polymeric stabilizers have been used for some time. The polymers typically used are styrene/acrylic resins prepared by solution or bulk polymerization in the presence of a chain transfer agent, with relatively high acid values of 140–300 mg KOH/g and average molecular weights between 1,000–5,000 g/mol. The carboxylic acid groups of these polymers are neutralized with a volatile base and dissolved in water. A typical problem associated with the use of these high acid value polymeric stabilizers is the inherent water sensitivity of coatings made from these dispersions. Polymeric stabilizers with much lower acid values can be synthesized through a neutralization and dissolution step followed by an emulsion polymerization process. Polymeric stabilizers form a "shell" around the core particles, functionally low molecular weight in acid. This shell will provide hydro-plasticization properties that effectively reduce the MFFT of the main polymer as long as neutralizing amine is present.

New Hybrid Acrylic Dispersions

Allnex has developed a new and proprietary emulsion polymerization process that uses an acrylic copolymer with gradient-type morphology and low molecular weight acid-functional stabilizers on the polymer particles. This "hybrid" process allows much better control over the mechanical properties of the polymer and maintains the advantageous balance between MFFT and hardness.

Figure 1. Particle morphologies: core-shell and gradient morphology. In hybrid technology, a gradient particle is wrapped in a shell with low molecular weight acid functionality

An example of a binder synthesized using this technology is SETAQUA® 6799. This polymer was developed primarily for the joinery market and is characterized by good outdoor durability and dirt pickup resistance. These properties and long working time also make this binder attractive for the decorative market. Some of the resin properties of SETAQUA® 6799 are given in the table below.

Table 1. Properties of SETAQUA® 6799

Dynamic mechanical thermal analysis (DMTA) was performed on SETAQUA® 6799. The plot can be seen in Figure 2)

Figure 2. DMTA plot of SETAQUA® 6799 (at a frequency of 1 Hz)

One of the striking features when looking at the storage modulus E' is the very broad glass transition beginning at approximately -15°C, whereas the loss tangent (tan delta) peaks at a temperature of 88°C. This means the polymer is rigid but still extremely flexible at room temperature. As a result, coatings formulated with SETAQUA® 6799 will have superior mechanical properties such as blocking resistance and post-cure control resistance. It also has excellent film formation properties at very low VOC. To compare some important properties of SETAQUA® 6799 with a commercial reference (core-shell technology), a simple white pigmented formulation was used. This formulation is given below:

Table 2. White pigmented topcoat formulation

Test results are given in Table 3.

Table 3. Test results for white pigmented topcoat formulation

* 1 = poor, 10 = excellent ** 0 = poor, 5 = excellent It can be seen that blocking resistance is excellent, especially at high film thickness, as is hardness. The SETAQUA® 6799-based coating also has extraordinary resistance to hand cream. Leveling and working time are also extremely good. Depending on the additives used in the formulation, working time values of 15 to 20 minutes can be achieved. The special morphology of SETAQUA® 6799 is also reflected in improved outdoor durability. In Figure 3, gloss retention (60° angle) of white pigmented coatings in accordance with EN927-6 is given for coatings based on core-shell, gradient and new hybrid technology. SETAQUA® 6799 is clearly superior in gloss retention.

Figure 3. Gloss retention of pigmented coatings measured at a 60° angle in accordance with EN927-6

This superior aging behavior is also evident when SETAQUA® 6799 is used in a clear wood stain without UV stabilizers (Figure 4). This image shows three panels of pine wood aged under accelerated UV conditions. The relevant formulation is given in Table 4.

Table 4. Clear wood stain without UV stabilizers

Figure 4. Accelerated aging of a wood stain based on SETAQUA® 6799 without UV stabilizers (two coats, brush applied 200 microns wet)

The other properties of this wood stain have also been excellent. With regard to the joinery industry, where newly coated materials are wrapped in film during storage and transport, which can cause staining due to condensation, we observed a notable improvement in early water resistance and chemical resistance. Additionally, alkali resistance will reduce quality claims resulting from contact with alkali materials such as cement and plaster. Results can be seen in Table 5.

Table 5. Test results for a wood stain

* 1 = poor, 10 = excellent ** 0 = poor, 5 = excellent Performance in clear coating systems can be further improved by adding UV absorbers. We used a 1:1 mixture of 1% benzotriazole and a hindered amine light stabilizer at 60% solids content in butyl diglycol. This difference can be seen in Figure 5 in a system consisting of one coat of stain primer + one coat of clear topcoat (400 µm wet - back of panels is uncoated). On the left is shown an unexposed SETAQUA® 6799-based clear coating. The other panels in Figure 5 were tested for 2000 hours in accordance with EN927-6. From left to right, panels with commercial reference and clear coatings based on SETAQUA® 6799 without UV absorber solution and containing 1% and 2% are shown.

Figure 5.

Effect of UV absorber use in clear coatings. Left: SETAQUA® 6799-based clear coating without UV absorber, unexposed. Remaining panels: exposed for 2000 hours, QUV-A This shows that performance is considerably higher than the commercial reference. With the correct amount of UV absorber, the effect of outdoor exposure is almost imperceptible.

Conclusions

Using the latest developments in emulsion polymer design, it is possible to formulate environmentally safe, water-based single-component paints that meet all requirements for joinery and decorative applications. These coatings have extraordinary mechanical and chemical resistance properties with minimum binder requirements. Dirk Mestach Manager Liquid Resins Synthesis Allnex Netherlands BV Marcel Meeuwisse Manager EMEA Technical Service and Business Development Allnex Netherlands BV Ankie van Gorkum Senior Chemist Paints and Applications Division Allnex Netherlands BV Luc Sterckx Senior Chemist Paints and Applications Division Allnex Belgium NV/SA

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