With ever-increasing environmental regulations, the use of VOCs in paint systems is under constant pressure. The paint industry is currently adopting new technologies and reformulating its products to meet the deadlines of scheduled VOC legislation. VOC regulations of 50 g/L in California’s South Coast Air Quality Management District (SCAQMD) are driving the reformulation of oil-based concrete paints to water-based paints. It is a challenge to formulators and resin producers to bring user-friendly products that meet the performance level that the market demands.
There are various technologies used to provide concrete coatings, including acrylics (solventborne and waterborne), epoxy, urethanes, polyurea and hybrids. In recent years, decorative coatings for concrete have gained popularity and are primarily based on acrylic emulsions. However, many water-based concrete coatings on the market have VOCs >100 g/L, which can not meet the West Coast 50 g/L VOC requirements. Elastomeric acrylic-type binders can easily meet the VOC requirements, but are prone to high levels of dirt pick-up.
In its technical note on exterior wall coatings for concrete, the U.S. Army Corps of Engineers has identified five characteristics that coatings should possess to resist outdoor elements: ultraviolet (UV) stability; breathability; resistance to wind-driven rain; flexibility; and resistance to alkali and fungi.(1) This paper will focus on the performance attributes of a new line of fluoropolymer (fluorocarbon) waterborne hybrid materials based on poly(vinylidene fluoride) (PVDF) resins,(2) which offer a quantum improvement in performance in all these areas, in formulations meeting stringent West Coast 50 g/L VOC requirements.
New PVDF-Based Hybrid Latex Technology for Concrete Coatings
PVDF is a semi-crystalline fluoropolymer that is unique in its ability to be compounded with certain acrylic resins, such as poly(methyl methacrylate).(3) This property has been exploited for decades in the metal coating industry; in fact, the original PVDF grade for coatings, Kynar 500® PVDF, has been in continuous commercial production since its introduction by the Pennwalt Company in 1965.(4) PVDF-based fluoropolymer coatings have traditionally been available almost exclusively as solvent-based, factory-applied finishes on metal substrates. Commercial coatings based on 70-80 wt% PVDF/20-30% compatible acrylic are renowned for their excellent exterior weatherability, which is based in large part on the outstanding resistance of PVDF resins to water, to UV radiation, and to other forms of photochemical and environmental attack. When formulated with highly color-stable inorganic pigments, these PVDF-based binders routinely meet the 10 year South Florida “Superior” weathering requirements for the most durable baked architectural metal finishes.(5) Early test panels based on this technology, which have now been on continuous exposure in South Florida for as long as 40 years, continue to show minimal color fade and excellent stay-clean properties. This performance is likewise now corroborated by literally thousands of buildings around the world.
In recent years, new developments in PVDF hybrid latex technology have made it possible to extend the weatherability performance of baked solventborne PVDF finishes to low-VOC, waterborne coatings. The new technology permits the use of PVDF-based coatings in field applications, and on heat-sensitive substrates, which cannot be baked – all without the use of high volumes of solvents.(6) To achieve weathering performance comparable to solvent PVDF-acrylic systems, the water-based systems use proprietary technology to create an intimate blend of the PVDF and acrylic resins inside each latex particle – a process known as hybridization. These latex polymer dispersions are anionically stabilized, and can be combined with water-based pigment dispersions and formulated with common coatings additives designed for waterborne coatings, similar to the way acrylic latexes are formulated.
The mechanical properties and minimum film formation temperature (MFFT) of these dispersions can be adjusted according to particular application needs. For coatings for concrete, an excellent balance of properties can be obtained when the neat latex MFFT is in the range of 5-10 °C.(7) Formulations based on these materials, with VOC levels below 50 g/L, have good elongation at break (>200%) and good low temperature film formation down to 40 °F,(8) while maintaining very good stay-clean properties, and with outstanding color-retention properties.
Excellent Efflorescence Resistance
|Figure 1 Click to enlarge|
Efflorescence resistance is a critical requirement for concrete coatings. Concrete surfaces tend to be highly alkaline when fresh, or when damp or wet from the intrusion of water, and they are prone to efflorescence. Efflorescence resistance is the ability of paint to inhibit the migration of water-soluble salts from an alkaline substrate through the paint to eventually be deposited on the paint surface. When dried, the salt is observed as a white discoloration of the paint film.
The superior alkali resistance provided by waterborne PVDF coatings is dramatically evident in a representative comparison study comparing low-VOC paint formulations applied to Blockaid™ cementitious panels.(9) Blockaid is a cementitious dry powder block filler used to create exposure panels that simulate fresh masonry surfaces, which have a high pH (~12). This surface is ideal for testing a coating’s ability to stop efflorescence, surfactant leaching and other failures brought on by hydrolysis of the coating and the alkalinity of the substrate (i.e., color fade, blistering). Figure 1 shows an accelerated testing comparison on Blockaid board, comparing two commercially available premium acrylic latex paints to a color-matched waterborne PVDF-based paint.
The coats of the different formulations were applied to sections of the highly alkaline test panels and allowed to dry seven days, after which they were placed in a humidity chamber for 48 h. The waterborne PVDF-based finish has tolerated the highly alkaline surface much better than all of the acrylic-based paints. No obvious water-soluble salts are observed for the waterborne PVDF-based coating even when the humidity chamber exposure is extended to three weeks. An efflorescence combined with cyclic freeze thaw test was also run, where the panels were run 5 cycles under conditions of freeze (16 h at 0 °F), thaw (16 h at 75 °F) and 16 h in a humidity chamber at 104 °F. In this more aggressive test, the same results are observed – the PVDF latex coating shows much better efflorescence resistance than premium acrylic latex coatings at a comparable VOC level of < 50 g/L.
Excellent Color Fade and Chalk Resistance Over Fiber Cement
|Figure 2 Click to enlarge|
Another critical performance requirement for exterior concrete coatings is durability – that is, the retention of properties after exposure. Decorative durability refers to a paint’s ability to retain its aesthetic appearance on exposure to the elements. Outdoor exposure will break down almost all coatings, causing them to chalk, fade and discolor. For conventional paints, color fade due to chalking occurs when the polymers holding the paint together – also called the binder – are degraded by the action of UV light, water, oxygen, etc. Acrylic paints, like other kinds of paints (e.g., polyesters, urethanes), can show significant levels of color fade and chalking after 5-7 years of Florida exposure. The superior weatherability of coatings made with high levels of PVDF resins is based on the ability of the fluorocarbon resin to resist ultraviolet rays, water, temperature extremes, oxygen and atmospheric pollutants. As a result, similar weathering performance is seen both for the traditional baked solvent PVDF paints, and for the new water-based latex materials, when comparable levels of PVDF resin are used.
Figure 2 shows the color fade comparison of premium acrylic latex coatings and a color-matched PVDF water-based coating over primed fiber cement panels, after 2500 h exposure with very aggressive UVB-313 bulb radiation. Figure 3 shows the color change data as a function of the cabinet exposure time. Within 600 h, the acrylic controls have significantly changed color. For the color-matched water-based PVDF hybrid coating, the color remains almost unchanged, even after 2500 h exposure. In the field, the color fade for darker paints is typically associated with chalking. In the UVB-313 cabinet as well, the acrylic paints were found to chalk dramatically, whereas the PVDF hybrid paint did not show chalking.
|Figure 3 Click to enlarge|
Figures 4 and 5 show another accelerated testing comparison on fiber cement boards, comparing a semi-transparent acrylic stain to a color-matched waterborne PVDF-based stain.
|Figure 4 Click to enlarge|
On the left is a commercial premium acrylic stain, and on the right is a PVDF hybrid-based stain. After 6000 h QUV-B (313 nm) exposure, the total color change for the acrylic sample (left) was measured to be about four times higher than for the PVDF hybrid-based paint (right).
Water Pick-Up Resistance and Breathability of PVDF Hybrid-Based Coatings
For many applications on breathable substrates like concrete, it is important for the coating to resist or repel liquid water and yet be permeable to water vapor. Exterior concrete coatings should be able to withstand penetration by wind-driven rain, both to prevent water from entering the concrete and to prevent contaminants such as airborne particulates and acids from being carried into the substrate with water. At the same time, entrapped water should be able to leave the substrate, since entrapped moisture can lead to damage from cyclic freezing and thawing, and can result in coating failure.
|Figure 5 Click to enlarge|
Fluoropolymer resins like PVDF hybrid products are inherently hydrophobic, and when compared with acrylics show much lower water pick up. The ability of the topcoat not to absorb water will improve the chances of keeping the substrates dry. Figure 6 shows the water uptake behavior for a PVDF hybrid latex dispersion paint formulation compared to several commercial trade sales latex paints. The steady state water absorption levels for the PVDF hybrid latex dispersion paint are in the 1-3 wt% range, very low compared to the all-acrylic latex paints, which have long-term water pickup values in the 8-20 wt% range.
|Figure 6 Click to enlarge|
The breathability (water vapor permeability) of concrete coatings can vary widely depending on the specific composition and material proportions of the coating product. A coating with a lower pigment-to-binder ratio (typically a glossy coating) tends to be less vapor permeable than a coating with a higher pigment-to-binder ratio (typically matte or flat coating), and a solventborne coating will tend to be less permeable than a waterborne coating with the same pigment-to-binder ratio and gloss level. In addition, the different polymers or resins used as binders in coatings have different inherent permeability values, and thus affect the breathability of the coating product in which they are used.
|Figure 7 Click to enlarge|
We have found that for PVDF-acrylic latex dispersion coatings, it is possible to tune the water vapor permeation rate over a large range, simply by adjusting the pigment volume concentration (PVC) of the coating. Figure 7 shows one example of this. For the PVDF waterborne coatings, with increasing the PVC level from 12% to 30%, the permeability varies from 10 to 80 perms while the equilibrium water pickup for this series is nearly unchanged across the range of PVC values tested, in the range of 1.3-2.5 wt% (not shown). Therefore, depending on the requirements of the application, the paint’s water vapor transmission properties can be independently adjusted across a wide range, by varying the pigmentation level, while still keeping a very low water pickup.
Dirt Pick-Up and Mildew Resistance of PVDF Hybrid-Based Coatings
|Figure 8 Click to enlarge|
Coatings based upon PVDF resins show very good resistance to dirt pick-up, and to the growth of mildew and other biological organisms, as demonstrated both in accelerated lab tests and also in real life field performance. Figure 8 shows a side-by-side comparison of PVDF hybrid-based white coating and a conventional acrylic white coating after 8 months in Florida south facing 5° angle exposure. These particular formulations were made without mildewcides, so as not to obscure the effect of the resin binder on these important properties. The panel on the left, made with a PVDF hybrid latex dispersion formulation, has stayed very clean while the panel on the right, which is a conventional acrylic roof coating formulation (but without added biocide), shows substantial dirt pick-up and biological growth.
A laboratory test of dirt pick-up resistance was also run. In this test, the paints were dried for one week, and then an iron oxide pigment slurry was applied and allowed to dry 3 h. The dry iron oxide was then rinsed off the paint with water and light wiping with a piece of wet cheesecloth. The panels were cleaned until no more iron oxide was being picked up by the cheesecloth, then the color difference (Delta E) was read between the area coated with the iron oxide slurry and a control area with no iron oxide slurry. In this test, larger Delta E numbers indicate that more iron oxide pigment is left on the surface of the paint. In this laboratory dirt pick-up resistance test shown in Figure 9, the PVDF hybrid-based paint shows significant lower Delta E number than all the acrylic-based white paints, again highlighting the “stay-clean” properties of these fluorocarbon coatings.
|Figure 9 Click to enlarge|
The “stay-clean” property is particularly important for concrete roof coatings. The excellent dirt pick-up resistance of PVDF hybrid latex coatings helps to maintain the highest level of solar reflectivity of roof coatings. By dramatically reducing surface roofing temperatures, building air conditioning costs are reduced.(10) The poor dirt retention over time of acrylic latex coatings can lead to a significant decline in total solar reflectance. For some white acrylic-based coatings in the field, with an initial TSR of over 70%, drops in the TSR (total solar reflectance) down to as low as 20-30% in 2-3 years have been reported.(11) The new PVDF hybrid waterborne technology can offer very high TSR retention. The test panels have a very high initial TSR of around 0.80, and more importantly retain over 95% of this value after six years South Florida exposure.(6)
The development of PVDF hybrid latex technology now allows the coatings industry to take advantage of the properties of the PVDF fluoropolymer for field and factory-applied coatings on concrete and other cementitious substrates. A grade with an MFFT value in the 5-10 °C range can be formulated into high-performing, low-VOC paints meeting stringent Southern California VOC regulations. Because these coatings are low VOC to begin with, and are extremely long-lasting, repainting is not needed nearly as often. With a longer service life, the cumulative amount of VOC generation is reduced even more, thus providing for an extremely “green” coatings solution. The ability of PVDF-based coatings to resist UV degradation, water and chemical attack, allows these coatings to more easily resist dirt, staining and mildew/algae growth. High-performance, low-VOC water-based coatings based on PVDF latex dispersions open up new opportunities to further extend the longevity, and performance, of concrete buildings and structures.