New Ultra-fast-drying Polyacrylate Resin for Two-component Automotive Touch-up Applications

Introduction

Fast-drying paints are becoming increasingly important in today's market. Particularly in the vehicle refinish sector, the ability to process repaired cars as quickly as possible translates directly into economic benefits. For this reason, there is continuous demand for shorter drying times at an ever-increasing rate. However, most fast-drying paints generally experience problems due to unstable other properties such as low early hardness, poor chemical resistance and short pot life. Allnex now presents Setalux 1226 SS-60, a new, state-of-the-art resin that offers ultra-fast drying time with virtually no debate, good pot life and excellent chemical resistance. For formulators of complete paint systems (i.e., primer, base coat and clear coat), another important benefit of this resin is the possibility of using it not only in the clear coat layer but also as the main binder in the primer layer. When formulated as a primer, it will provide very rapid sandability, excellent adhesion to many substrates and optimized pot stability. Clear Coat Results Based on Setalux 1226 SS-60 Setalux 1226 SS-60 is an extremely fast-curing, hydroxy-functional acrylic polyol for two-component vehicle refinish clear coat and primer applications. This resin has been designed to be easy to formulate. Minor paint optimization will be required to develop a balanced paint formulation with excellent properties. Some basic resin properties are listed in Table 1.  

With fast-drying systems, it is generally very difficult to find a balance between drying time and pot life. For example, the chemical reaction that needs to be accelerated to achieve fast drying (Reaction I) is the same reaction that needs to be slowed down for long pot life. Setalux 1226 SS-60 has been designed to provide both.

  Furthermore, achieving good early hardness with fast-drying coatings is generally difficult. Fast drying is usually achieved by increasing the reaction rate of Reaction I. However, this will also result in early vitrification of the molecules in the coating by significantly reducing their mobility. For fast-drying coatings, vitrification generally occurs before all solvent has evaporated. As a result, solvent molecules will be less mobile after the coating has vitrified and will function as plasticizers in the dry coating (Figure 1). Particularly during the early period after drying, this normally reduces hardness noticeably. Of course, a formulator has the possibility of using faster evaporating solvents, but this is generally accompanied by poor appearance. The balanced resin design of Setalux 1226 SS-60, in other words, allows for the combination of very rapid drying with excellent early hardness and outstanding appearance. This formulation can be diluted to 16 seconds Din-cup 4 with the reducer component (butyl acetate). It can be sprayed using an HVLP spray gun at 150–250 kPa spray pressure and subsequently dried at 60°C for 30 minutes or at ambient temperature. The most important coating properties obtained with this formulation are shown in Table 3 and are comparable with conventional systems.  

The data in Table 3 clearly show that the Setalux 1226 SS-60-based clear coat dries significantly faster when compared with conventional competing products, almost by an order of magnitude. This is coupled with a much improved pot life—drying balance and increased early hardness development.

All of this will result in economic benefits when used as a vehicle refinish clear coat, as the car can be processed hours earlier. Another advantage of fast drying and good early hardness is the early development of chemical resistance. Given that most two-component isocyanate coatings develop chemical resistance only in the first two weeks after application, Setalux 1226 SS-60-based coatings formulated according to Table 2 will achieve excellent chemical resistance within the first day after application. The performance properties of Setalux 1226 SS-60 listed in Table 3 are coupled with excellent durability, extremely good flexibility and mechanical properties. It is also worth noting that the appearance of Setalux 1226 SS-60-based formulations is significantly improved when compared with competing formulations. Particularly short-wave is much lower. This is achieved in part by the good leveling behavior of the combination of Setalux 1226 SS-60 with Modaflow Lambda.

Although good clear coats for vehicle refinish applications can be obtained using Setalux 1226 SS-60 as the main binder, this Allnex product can also be used as a modifying resin in two-component applications where resin blending is preferred. Here, Setalux 1226 SS-60 will improve the drying speed of the overall system.

This very good early performance of Setalux 1226 SS-60-based coatings results from very rapid isocyanate conversion and property development, as can be seen from the results below. Isocyanate conversion during drying at room temperature of a Setalux 1226 SS-60-based 2K PUR coating formulation—as defined in Table 2—is measured with the aid of FT-IR: Isocyanate conversion was monitored in real time by measuring the decrease in relative IR peak area at approximately 2270 cm⁻¹ using the carbonyl peak area at approximately 1725 cm⁻¹ as a reference. The same was done with a coating formulation based on Competing Product 1. The conversions measured in the first few hours of drying for both samples are shown in Figure 2, where it can be clearly seen that conversion proceeds much faster for the Setalux 1226 SS-60-based formulation than for the competing product: after approximately 14 hours of drying, the competing product-based formulation achieves only 60% while the Setalux 1226 SS-60-based product reaches approximately 85% conversion.

Dynamic Mechanical Thermal Analysis (DMTA) measurements were also applied to thin coating films (approximately 50 mm) in tensile mode, by heating the sample from –50°C to 150°C at 5°C/min after 1 day of room temperature drying, with a strain of 0.03% applied at 1 Hz frequency.

DMTA plots are reported in Figure 3; the measured Tg values and calculated cross-link densities (XLD) are given in Table 4. As we can see, the properties developed in the Setalux 1226 SS-60-based coating progress faster and nearly reach final properties after one day of drying at room temperature (a plateau is observed after the Tg change). However, in the coating based on the competing product, we can clearly see that after 1 day of room temperature drying, curing is not yet complete and further cure occurs during the DMTA measurement, beginning approximately 50 degrees after the Tg change. Consequently, these FT-IR data and DMTA data support the rapid curing of the Setalux 1226 SS-60-based coating as well as both very rapid hardness development and chemical resistance.  

Primer Results Based on Setalux 1226 SS-60

Setalux 1226 SS-60 can be used for VOC-compliant two-component primer formulations for vehicle refinish applications with rapid sandability, excellent adhesion to most substrates and optimized pot stability. The guide formulation is given in Table 5 and the main application data are summarized in Table 6.   This formulation can be diluted to 27 seconds Din-cup 4 with the reducer component (Butyl acetate 50 / Methoxy propyl acetate 40 / Solvent naphtha 10) to obtain a primer composition with a VOC content of 540 g/l and a 40% pigment volume concentration. The formulation can be sprayed at 300–400 kPa spray pressure and subsequently dried at ambient temperature to give a 25-minute tack-free primer that can be sanded after 1.5 hours and has very good adhesion to different substrates (plastic, aluminum, steel). Rheology has been optimized through the use of Additol XL 280. The use of this rheological agent in combination with Setalux 1226 SS-60 ensures a viscosity-stable paint product; the paint remains easy to mix and pour throughout its shelf life. Additionally, careful mixing in the can is not required. Good thixotropy of the paint prevents settling.  

Conclusion

Setalux 1226 SS-60 is Allnex's new, state-of-the-art resin offering ultra-fast drying. When used in clear coat formulations for vehicle refinish applications, it provides extremely early hardness and develops its chemical resistance very rapidly. In two-component primer applications, Setalux 1226 SS-60 will provide excellent sandability after 1.5 hours. This is accompanied by good adhesion over many substrates. The coating performance of Setalux 1226 SS-60-based formulations can be enhanced with the Allnex additive package, such as flow and leveling in clear coats and pot stability in primers. Allnex continuously develops technologically innovative resins with increasingly advanced performance. For this reason, the introduction of further ultra-fast drying resins is expected in future periods. Author: Elwin de Wolf / Laboratory Manager / Allnex Compiled by: / Kimteks Kimya Tekstil Ürünleri Tic. A.Ş.