By , , and , Evonik Corporation, USA
As the coatings industry moves towards lower volatile organic content (VOC) or near-zero VOC, achieving good film formation in waterborne systems without sacrificing other coating properties has become challenging. Low-VOC coatings are softer and tackier due to the use of low Tg resin, which makes them more susceptible to capturing dirt and dust, especially in warmer and more humid climates. Of all the tradeoffs in low-VOC formulations, dirt pick-up resistance is one of the most noticeable changes, particularly in traditional exterior house paints. Although dirt pick-up is a complex process, the interaction of the resulting paint film with environmental particles, weather, and other variables is ultimately defined by the surface properties, which affect the accumulation of dirt. This study will explore the effects of siloxane and silica additive technologies on dirt pick-up resistance using an accelerated test method, and the mechanism of action of these additives will be discussed.
Introduction
Dirt pick-up resistance (DPUR) is a topic of high interest in exterior architectural coatings. Consumers want paint that can resist staining while also being easy to clean. In developing dirt-resistant paints, outdoor exposure is necessary for providing real-world data; however, these tests can take many months, or years, before a coating’s performance can truly be understood. Hence, accelerated DPUR tests are critical to enabling formulation development and providing insights into how various components may contribute to the DPUR of a coating. Unfortunately, there is currently no standard accelerated DPUR method in the coatings industry; several methods focus on a dry deposition of a dirt source, meanwhile methods developed in wetter climates feature dirt application using a dirt/water slurry.1 In general, existing accelerated DPUR methods follow a procedure consisting of 1) applying the paints to panels, 2) curing the coated panels under conditions of controlled temperature and humidity, 3) exposing the cured panels to ultraviolet (UV) irradiation and higher temperature, 4) applying and removing the dirt from the conditioned panels, and 5) analyzing the extent of dirt pick-up and removal from each panel. However, the dirt application and removal step has been acknowledged to be the greatest source of low reproducibility for these methods.2 In this work, a new accelerated DPUR test method was developed to facilitate studying both initial pickup of dry dirt and how well that dirt could be rinsed off when subjected to a rinsing process.
While siloxanes have traditionally been utilized to enhance surface slip, flow, and leveling,3 recent research has demonstrated that siloxane surface control agents can also significantly boost block resistance of coatings.4,5 In a similar vein, spherical silica particles have been found to increase burnish and wet scrub resistance of architectural coatings due to their effect on the dry coating film.6 These results motivated us to investigate the impact of various polyether-modified siloxanes, emulsions of higher molecular weight and crosslinked siloxanes, aqueous dispersions of fumed silicas, and spherical precipitated silicas on dirt adhesion and its release.
Experimental
A commercial low-VOC waterborne, self-crosslinking acrylic exterior satin paint was chosen to evaluate various siloxane and dispersed silica additives. Siloxane-based surface control additives (SCAs) were post-added at 0.50 wt % to the commercial paint, mixed well, and the paints were allowed to stand overnight before use. Similarly, dispersed silica additives (DSAs) were post-added at 1.0 wt % to the commercial paint, mixed well, and the paints were allowed to stand overnight before use. The characteristics of the additives tested are summarized in Tables 1 and 2.
A low-VOC waterborne, self-crosslinking acrylic exterior flat paint with a 76% PVC was formulated (Table 3) and it was used to evaluate the effects of full volume-to-volume replacement of calcium carbonate filler with spherical silica fillers (SPHs) of varying particle size (Table 4). Similarly, once mixed, the formulations in Table 3 were allowed to rest overnight to release residual foam from mixing.
The commercial paint samples to which had been post-added various siloxane and dispersed silica additives and the coating formulations in Table 3 were applied to scrub charts using a 150-mm wide bird-type film applicator for a 4-mil wet film thickness. After two days of drying in a standard conditioning atmosphere of 21–25 °C and 45–55% relative humidity (ASTM D4332-22), the scrub charts were trimmed to the test panel size (12.7 cm x 10.8 cm; center of coating) and cured in a Q-SUN Xe-3 Xenon Test Chamber (Ser. No. 20-34603-79-X3HBSE; from Q-Lab) at 40 °C and 45% relative humidity using a full-spectrum sunlight filter with an irradiance of 0.89 W/m2 at 340 nm for three days. The test panels were removed and allowed to rest for two hours before dirt deposition. A number of studies have revealed that typical outdoor dirt particles that soil exterior coatings have a number median diameter of approximately 100 nm.7-9 Hence, for this study, Lamp Black 101 (carbon black from Orion Engineered Carbons), and BAYFERROX® 509 (iron oxide from Lanxess) were chosen. These dirts were applied via fine-mesh sifters to achieve a complete and uniform coverage of the test panels. After one day, the dirt was removed by lifting the panels upright and tapping 15 times against the benchtop. The panels were then rinsed at an upright position with 15 misting sprays of DI water from a spray bottle at a distance of 20 cm. The residual water was removed by tapping the panels 15 times, after which they were returned to a horizontal position and dried overnight.
Continue reading in the March-April digital issue of CoatingsTech