Surface coatings give mechanical protection as well as optical and financial appreciation to surfaces of any kind. Dirt pick-up or bio-fouling of coatings are accordingly not only optically adverse progresses but lower the value of the coated object. To minimize the loss of protection and depreciation due to dirt pick-up and bio-fouling, coatings are renewed on a regular basis, with accompanied costs for material and labor. Here, an improvement of the coating and a resulting cleaner and more resistant surface for a longer period of time would give a longer service life and thus reduced costs for the user. This paper reports strategies to decrease dirt pick-up and bio-fouling of coatings with the addition of silica particles.
Bio-fouling and soiling are expensive for society. From bacterial adhesion on medical implants that cause complicated infections to barnacle adhesion on tankers that increase the friction to water leading to higher fuel consumption. Adhesion of soil on a painted house downgrades the optical impression and accordingly the value of the property.
Hence a development of coatings providing true anti bio-fouling and/or anti dirt pick-up properties will decrease costs for society in the long run and provide a market opportunity for the coating company.
Any surface, whether natural or synthetic, is coated initially with local environmental constituents such as water, electrolytes and subsequent organic substances. The presence of this conditioning film can provide the impetus for microbial growth and further colonization. Microbial adherence and biofilm production proceed in two steps: first, a reversible physical attachment to the surface, followed by a second irreversible chemical step, involving the multiplication of cells and the synthesis of an extracellular polymeric film.
As for the soiling of surfaces, dirt particles are literally everywhere, spread around by wind and rain, or, in the case of the finest particles by Brownian movement. However, the type and amount of dirt particles that come into contact with the exterior coating at the weathering location vary a lot and this naturally has a determining effect on the dirtiness of the coating. Hence coatings in cities or industrial areas (having a higher concentration of suspended dust in the air) become dirtier after shorter time than coatings in a rural environment with a lower overall dust concentration.
To address the problem of soiling and fouling, there are several approaches that can provide guidelines for future development yielding tomorrow’s competitive solutions with improved bio-fouling and soiling performance:
• Paint surface hardening
• Prolonged effect of existing biocides
• Novel biocides
• Self-cleaning coating via superhydrophobic mechanism
• Paint erosion
• Weak dirt adhesion
• Minimize nitrogen content in coating
• Reduce oxygen level in water
• Block enzyme activity which enhances micro-organisms glue production
• Introduce enzymes degrading bio-foulers glue
• Avoid surface recognition
• Use secondary metabolites
• Manipulate the microorganisms communication
• Add a cationic polymer at the surface
This paper elaborates and discusses experimental approaches of the top three methods and presents results on how to improve dirt pick-up and bio-fouling performance.
As mentioned above the value of a property can decrease if the painted surface looks dirty. The consumer owning the property might need to repaint more often which yields costs for material and labor or is time consuming (if the consumer paints himself). It is thus an interest from paint companies to make paints which are more durable against dirt pick-up. The aim of this project was to understand which factors are important for dirt pick-up; or – in other words - increase dirt pick-up resistance. For this purpose we have developed methods in the laboratory to evaluate dirt pick-up and tested several parameters which were expected to influence the soiling. Lab results were compared with exterior testing.
The dirt pick-up test was based on our industrial experience, spraying the test panels with an aqueous solution of a model-dirt mixed according to Scheme 1. The dry components were mixed in a mortar and then the pitch was added and stirred in manually. 1 g of the dirt was mixed with 1 g butyl glycol and these 2 g were filled in a spray bottle and filled up with 998 g de-ionized water. The dirt solution was filled into a pressurized aerosol bottle and sprayed on the samples, set up in a 45° angle, as shown in Figure 1. The samples were sprayed with the dirt solution for 3 times with one minute between each spraying. The distance between the spray bottle and the samples was 30-40 cm. The maximal possible pressure (≈ 3 bar) of the bottle was used to perform each spraying. After the contamination the samples were left for 1 day for drying followed by rinsing with de-ionized water. To evaluate the dirt pick-up of the surfaces, the L- value (a brightness index) was measured before and after the contamination as well as after rinsing.
Several types of paints were evaluated with this setup with various binder chemistries painted on metal panels. The painted panels were tested fresh and weathered in a weatherometer (corresponding to 6 months of ageing in southern Sweden). ΔL values for 22 samples are shown in Figure 2.
Formulation 10 and formulation 15 missed on purpose a cross-linker and were therefore expected to be very tacky with an expected high dirt pickup. 10w and 15w have probably been washed away during weathering. The “normal” formulations 20-65 can roughly be divided into two groups:
(A) high dirt pick-up 55, 55w and 65
(B) moderate dirt pick-up 20, 20w, 25, 25w, 35, 35w, 40, 40w, 45, 45w, 65 w.
To measure the tackiness of the coatings after drying, the adhesion of a hydrophobic silica particle to the surface was measured with AFM colloidal adhesion technique. The probe used was a silanized, hydrophobic silica particle. We measured the force needed for the probe to detach from the coating surface. Thus, it is a measure of the adhesion force and thus the tackiness of the surface.
AFM measurements show that formulations 10 and 15 are very tacky. We categorized the results into high adhesion (> 6 mN/m) and low adhesion (< 6 mN/m). That way, we can divided the samples 20-65 into two groups:
(A) high hydrophobic adhesion 40, 40w, 50w, 55, 55w, 65
(B) low hydrophobic adhesion 20, 20w, 25, 25w, 30, 30w, 35, 35w, 45, 45w, 50, 65w
For most of the paints the ranking between high dirt pickup and hydrophobic adhesion is fairly good. Only formulation 40 does not follow the rule. A simplistic explanation could be that that the dirt pickup seen by eye is mainly linked to carbon black giving the black appearance changing the lightness value L before and after contamination. Carbon black is hydrophobic in nature and that would then agree with the adhesion strength of a hydrophobic colloidal probe.
Based on these data and the exterior testing results (data not shown) we decided to evaluate the possibility of reducing dirt pick-up by reducing the tackiness of the paint film. The idea was that by adding nanoparticles (in this case silica nanoparticles) they would to some extent move to the air-coating interface and induce a harder and less tacky coating surface.
Anti-soiling effect due to harder surface induced by nanoparticles
To evaluate the possibility of decreasing dirt pickup of coatings with the help of surface hardening nanoparticles, alkyd binders, acrylic binders and mixtures thereof we enriched with hydrophilic (Bindzil 40/220, average diameter 12 nm) or amphiphilic (CC40, epoxy silane modified, average diameter 10 nm) silica nanoparticles. The addition of the nanoparticles had a strong influence on the microstructure of the coatings, as shown in Figure 4.
Furthermore intendation tests using a microintender show that coating films containing silica particles are harder than films without particles. Another feature is that keeping the intender at a fixed load results in a bigger creep in the latex coating compared to the coating containing silica nanoparticles. The impact of 25 wt% nanoparticles on the surface properties of a latex coating is shown in Table 1.
The above data gave the impetus of the idea that addition of silica nanoparticles in a commercial paint might reduce dirt pickup of the paint. Paints for dirt pick-up evaluations were formulated with 3 wt%, 6 wt%, and 12.5 wt% silica nanoparticles according to Table 2.
We investigated commercial paints based on alkyd binders, acrylic binders and mixtures thereof. We made two types of modification of the commercial paints:
(A) addition of 10 nm silica particles (40/220)
(B) addition of silane-modified 10 nm silica particles (CC40).
The latter type of modification makes the particles more amphiphilic in character compared to the hydrophilic particles.
The previously described laboratory dirt pick-up test was modified to include a heating step at 50 °C for 1 h after each contamination cycle. In this way inclusion of dirt into the paint due to binder movement is included in the result. A representative chart for the impact of silica amount is shown in Figure 5 for an alkyd coating. Generally the dirt pickup is reduced with higher amounts of silica present in the formulation. Note that all formulations are painted the same day as they are produced. This is important to be aware of since hydrophilic silica would flocculate the paint upon storage.
Figure 6 shows a summary of the results after three contamination cycles for different binder systems without silica particles and with 12.5 wt% CC40 or 40/220. For almost all paints the dirt pickup is lower for silica-modified paints compared to the commercial paints. This indicates that the change in microstructure and harder surface reduce the dirt pickup.
To further investigate the correlation between silica particles and dirt pick-up we placed 9 replicas of each paint and silica-modified paints outdoors for 9 months in Bogesund, Sweden, as shown in Figure 7.
The painted wood panels have been distributed at random over the rack. The rack is placed 45° facing south to increase paint degradation by sun light. The results of this weathering experiment are shown in Figure 8. From these data it is clear that the silica nanoparticles formulated into the paints help to reduce dirt pickup in three of the four tested paints. This finding can provide the possibility for reduced dirt pickup by reformulation of the commercial paints available today.
Another important issue for coatings is so called bio-fouling – fungi and other micro- and macro- organisms grow on or into the coating – degrading and uglifying the surface, as shown in Figure 9.
Classical solutions to avoid growth of fouling species on land under water cannot longer be used due to legal restrictions. For example tributyl tin (TBT) was banned in 2003 for marine applications. TBT is poisonous to several marine species but very effective for antifouling purposes. In Europe, there are several new regulations coming up and one of the toughest is the biocide directive BPD. The consequence of the biocide directive is that the most efficient substances against mold and algae are being phased out. The substances that are allowed are efficient but leak out to quickly from today’s coatings leaving it susceptible for biological growth including mold growth. Several strategies to avoid or minimize fouling can be thought of. Here we will limit ourselves to the biocide approaches.
As can be seen in Figure 9 biocides are very effective to suppress growth of organisms - no fouling is found in the left image using the novel biocide in a marine paint. On the other hand not using a biocide leads to the colonization of among other things barnacles. It must be kept in mind that fouling and barnacles can severely increase the cost and environmental impact for shipping in terms of increased fuel consumption.
One major drawback with new molecules is a costly procedure for testing that must be conducted before application in Europe. Hence it might be of interest to use existing molecules in smarter ways. At YKI, the Institute for Surface Chemistry a large research effort is performed in order to control the release rate of actives in various industrial areas. One example of a carrier/host of actives such as biocides is the use of mesoporous particles. The particles have a well-defined pore size that can be precisely controlled in the region 2-15 nm. Figure 10 shows transmission electron microscope (TEM) images of mesoporous particles produced in a spray reactor at YKI. These particles can be loaded with actives that are released over longer periods of time.
To evaluate the possibility to use those particles for sustained release of biocides in coatings, we loaded encapsulated mesoporous particles with the commercially available biocide OIT. Exterior acrylic emulsion paints were prepared from these particles and compared to standard formulations including OIT and a reference formulation without the biocide. The paints were applied on filter paper and subjected to two weathering cycles in a weatherometer. The coated filter paper samples were placed in Petri dishes containing agar and tested against growth of three different molds (Aspergillus Niger, Cladosporium sphaerosphermum and Penicillium funiculosum). Figures 11-13 show the results of these experiments, showing a newly painted sample and to the left a sample subjected to two weathering cycles.
As expected the paint with no biocide cannot withstand mold growth even on a freshly painted surface. The state-of-the-art formulation loses quickly its performance - probably due to too fast leakage of the biocide from the paint. The paint with the biocide encapsulated in coated mesoporous particles show perfect performance after two weathering cycles. The result indicates that a slower release rate of the biocide into the coating can enhance the durability of paint against mold growth.
In a series of experiments we could correlate dirt pick-up with surface hardness and tackiness. A silica particle-induced toughening of the paint surface showed decreased dirt pick-up in three out of four commercial coatings and is thus seen as a promising approach.
To meet new and/or upcoming legal restrictions on available biocides to fight bio-fouling, we have successfully shown that meso-porous silica particles can improve the long term performance of alternative biocide formulations.
About the authors
Jens Voepel is project manager for polymeric materials and polymer synthesis at YKI, the Institute for Surface Chemistry in Stockholm, Sweden. Jens holds has a PhD in Polymer Technology from the Royal Institute of Technology (KTH) in Stockholm (June 2011) and a Bachelor of Science in Chemistry and Materials Science from The University of Applied Science in Rheinbach, Germany. Jens joined YKI in March 2011 and is gaining experience with project related to coatings, controlled delivery and pigments. He can be contacted at Jens.Voepel@yki.se.
Anders Larsson is responsible for Business Development of Advance Materials and Area Manager Coatings at YKI, the Institute for Surface Chemistry in Stockholm, Sweden. Anders has a PhD in Physical Chemistry from Gothenburg University (GU) in Gothenburg (1999). After doing his postdoctoral studies at the Max Planck Institute for Colloids and Interfaces (Germany) he joined YKI during year 2000. He has led several projects related to coatings such as film formation studies, development of cleaner surfaces, controlled delivery of biocides and much more. He can be contacted at Anders.Larsson@yki.se.