James T. Connaughton, Joseph M. Rokowski and Thomas Ennis Rohm and Haas Company Springhouse, Pennsylvania ABSTRACT Acrylic polymers are widely used in the manufacture of low and steep slope roofing materials and in field-applied coatings. They are used to coat reinforcing mat for asphalt roll goods and as modifiers for concrete roof tiles. They are also used as binders in OEM coatings for concrete, fiber cement and metal roof tile, and field-applied coat¬ ings for modified bitumen and EPDM thermoset single ply membranes. This paper, presented in two sections, describes their use, benefits, and limitations in these specif¬ ic applications. The first section describes the use of acrylic polymers in the formulation of sealers for the surface of concrete roof tile. The sealer is designed to prevent efflorescence, an inorganic reaction by-product of the hydration of the Portland cement used to manu¬ facture the tile, often observed as a whitening of the tile surface. Tiles can sometimes exhibit other surface appearance phenomenon such as blushing and sealer cracking. This section describes these phenomena and provides techniques for diagnosing these appearance objections and offers suggestions to help restore the appearance of the tile. The second section of the paper describes the use of acrylic polymers in coatings for APP and SBS modified bitumen and EPDM single ply roofing, both when the roof is installed and during its life as a maintenance regime. Typically, these coatings dramat¬ ically improve the solar reflectance and prolong the roof life. This section describes the weathering performance of coated versus uncoated membranes. Mechanisms for mem¬ brane degradation and coating protection are elucidated using data from laboratory and field tests. Information needed by the roofing consultant to understand the benefits as well as the limitations of these acrylic-based roof coatings will be provided. James Connaughton James Connaughton has over 14 years’ experience working with synthesis issues and research on architectural and industrial coatings. Much of that time was spent dealing with the development and application of acrylic polymers and coatings for concrete and metal roof tile, fiber cement, and gypsum materials. Areas of particular interest include early development properties and long-term exterior durability. Joseph Rokowski Joseph Rokowski has been involved with coatings for 20 years. He specializes in adhesion issues, exterior durability matters, and roofing substrate analysis. Connaughton, Rokowski and Ennis—27
INTRODUCTION To those in the concrete roof tile industry, perhaps nothing is more troublesome than the appearance of efflorescence or “whitening” on newly manufactured and installed roof tiles. The presence of “whitening” on col¬ ored tiles can ruin, at least in the short term, the beauty and aesthetic potential that concrete tile roofscapes were designed to deliver to homes and commercial structures regardless of their architectural style. However, whiten¬ ing on concrete tile roofs is not always caused by efflo¬ rescence. There are other phenomena that can occur in the manufacturing process that are often misdiagnosed as efflorescence. This paper will focus on identifying and treating the three most common causes of whitening on concrete roof tiles: efflorescence, blushing, and sealer cracking. Efflorescence Efflorescence is a naturally-occurring phenomenon resulting from the reaction of Portland cement and water, otherwise know as the hydration of cement. It can be found wherever cement or mortar is used, such as high¬ ways and foundations for homes; or more decorative applications, such as concrete roof tiles and patio blocks and pavers. In the case of commercial applications, efflo¬ rescence is not very noticeable to the untrained eye against the common, gray-colored concrete. However, on decorative pieces which tend to be colored with pigments for aesthetic purposes, the presence of efflorescence detracts from the original appeal of the product and quite often leads the purchaser to believe that there is some underlying problem at fault. Differentiating Between Primary and Secondary Efflorescence In simple terms, the major difference between prima¬ ry and secondary efflorescence is what time during the product lifecycle that each phenomenon presents itself. Primary efflorescence generally occurs during the first few hours after the cement is mixed and formed into its intended shape. When cement (which is predominately calcium silicate) is hydrated, it produces calcium silicate hydrate, the compound which bonds the sand and other ingredients together and gives the strength and durability to the final concrete product. The hydration reaction also produces (as a by-product) calcium hydroxide, which, when in solution, is able to travel through capillaries to the surface of the concrete. It is when the calcium hydroxide reacts with atmospheric carbon dioxide, either below or on the surface of the concrete, that the insoluble white deposit known as calcium carbonate or efflores¬ cence is formed (Figure 7). Ca(OH)2 + CO2 H2O + CaCO3 (insoluble) CafOHjj + CO2 — ► CaCO3 + H2O CafOH^ Figure 1: Schematic of Formation of Primary Efflorescence in Presence of Excess Water. In decorative products (such as concrete roof tiles and pavers in which product shape and concrete quality is improved by compaction of the water-hungry concrete in or on molds), the presence of capillaries and voids is minimized, thereby limiting the pathways that efflores¬ cence takes to the surface. For this reason, efflorescence in these types of products is usually contained at or just below the surface of the capillaries rather than appearing as a whitish layer of crystals over the entire surface of the tile (Figure 2). Even in this relatively inhibited state, efflorescence can still have a very damaging aesthetic effect on the final product by lightening the intended color derived from the organic pigments and diminishing the much sought after color intensity. Furthermore, mois¬ ture in the form of rain or dew can work to disperse the salts at the surface of possibly unplug the capillaries of “green” tiles, thereby leading to contamination of the entire field. Unlike primary efflorescence, secondary efflores¬ cence may occur for weeks or months after the manufac- Connaughton, Rokowski and Ennis—29 ishing process may occur over a period of years. Of par¬ ticular note is the case of sodium sulfate, which, depend¬ ing on the temperature and humidity, can exist in the anhydrous and hydrous state. The constant changing of this salt from one state to the other within the concrete matrix can create forces that produce “spalling,” an unsightly, blistering-type effect on the surface of the con¬ crete. Figure 2: Formation of Primary Efflorescence in Highly Compacted Concrete with Minimum Water. turing process as the concrete continues to cure. And unlike primary efflorescence, which tends to be uniform over the surface of the tile or paver, secondary efflores¬ cence may occur sporadically and randomly as long as water can penetrate the capillaries and soluble salts are present in the body of the concrete product, creating a blotchy and mottled appearance on the surface (Figure 3). In this case, the soluble salts are present as a result of the continuing curing process and are once again carried to the surface with the flow of moisture where carbona¬ tion takes place. Water Figure 3: Mechanism of Secondary Efflorescence. It should also be mentioned that other soluble alkali salts such as the carbonates and sulfates of sodium, potassium, and magnesium, along with the chlorides of calcium and other metals, may also be deposited on the surface. In areas of high precipitation, these salts will be continually washed away and reformed until the supply of salts within the concrete that comes in contact with migrating water is exhausted. In areas where the moisture supply is low, it is possible that this washing and replen- Finally, even though the calcium carbonate deposits of both primary and secondary efflorescence are insolu¬ ble in water, it should be remembered that they are “tem¬ porary” in nature. Over time and in the presence of water and carbon dioxide present in the air, the calcium carbon¬ ate will be further carbonated to soluble calcium bicar¬ bonate, which will eventually be washed away by rain¬ fall. This step in the cement process may, however, take years to occur and rectify itself. CaCO3 + CO2 + H2o -*- Ca(HCO3)2 (soluble) Now that the mechanisms of efflorescence have been explained, it is possible to look at the role acrylic sealers play in the prevention of efflorescence on concrete prod¬ ucts. Preventing Efflorescence with Acrylic Sealers To those companies involved in the production of concrete products, the minimization of efflorescence has undoubtedly been well studied over the years. Methods that have evolved and are practiced include but are not limited to changing the proportion of the mixture, using special cements and aggregates, the use of chemical agents such as stearates or silicones to react with or tie up available calcium hydroxide, and changing the process of the mixture through curing or by the introduc¬ tion of excess carbon dioxide. However, none of them have achieved the widespread acceptance nor have the ability to prevent efflorescence without inhibiting the strength and durability of concrete as have 100% acrylic sealers. Acrylic sealers are used throughout the world on a variety of cementitious products due to their ability to provide long-term durability and protection against weathering and UV attack. Whether used as solvent¬ based or as water-based coatings, 100% acrylic polymers are transparent to ultraviolet light, which makes them the logical choice for exterior exposures. However, because of the ever increasing regulatory pressures found in much of the developed world, water-based acrylic sealers have taken the lead role in providing maximum efflorescence resistance on cementitious substrates. Using water-based Connaughton, Rokowski and Ennis—30 acrylic polymers as sealers during the manufacture and application of concrete products also allows for easy water cleanup of storage vessels, transfer pumps, lines, and spray guns, as well as greatly reducing odor of sol¬ vents in the plant. Preventing Efflorescence with Acrylic Sealers Acrylic polymers, when applied under the proper conditions and at recommended film thickness, prevent efflorescence by forming a thin, continuous film over the surface of the concrete substrate. The acrylic film blocks the capillaries, thus preventing the calcium hydroxide from reaching the surface of the concrete. Additionally, acrylic films, being breathable, allow carbon dioxide to pass below the film to react with calcium hydroxide and form a plug of calcium carbonate within the capillaries, further preventing the migration of the ingredients neces¬ sary for efflorescence from reaching the surface of the concrete (Figure 4 and Photo 1). Ca(OH)2 Figure 4: Schematic of How Acrylic Sealer Reduces Primary Efflorescence by Blocking Migration of Calcium Hydroxide to Concrete Surface While Allowing Carbon Dioxide to Pass Through to Form Plug of Calcium Carbonate in the Capillaries. AB CD Photo 1: Preventing Efflorescence with a Properly Applied Acrylic Sealer A) Concrete tile unfinished and unexposed. B) Concrete tile sealed properly and exposed 7 days 60 degree C water bath. C) Concrete tile sealed with poor quality sealer and exposed 7 days 60 degree C water bath. D) Concrete tile unfinished and exposed 7 days 60 degree C water bath. Further, the acrylic film prevents the penetration of moisture below the film surface, which is a necessary component of secondary efflorescence. In other words, the rain and dew that would otherwise migrate in and out of the capillaries as a result of natural weathering is kept above the concrete surface and the extraction of deposi¬ tion soluble salts does not occur (Figure 5). Figure 5: Schematic of How Acrylic Sealer Filin Prevents Secondary Effluorescence by Acting as a Water Barrier The Film Formation of Acrylic Sealers As mentioned earlier, the formation of a continuous film is necessary to achieve efflorescence resistance. With solvent-based polymers, films are formed with the evaporation of the solvent in which they are dissolved. With water-based polymers, film formation is more com¬ plex and takes place in three discrete steps. In Stage I, water evaporates from the latex surface, gradually forcing the spherical latex polymer particles into a closely packed array. In Stage II, initial coalescence occurs as interstitial water evaporates and the polymer particles undergo deformation due to interfacial pressures. Ultimately, this pressure leads to a transition stage in which the particles assume a closely packed hexagonal array. Finally, Stage III is reached in which the interparti¬ cle boundaries lose definition, and the polymer chains within each particle intermix to form a smooth, homoge¬ neous film. Figures 6 and 7 depict the film formation as idealized models. Figure 8 presents the film formation process on a time scale and shows the increase in solids of the film as it progresses through each stage. Note that initially the water loss due to free surface evaporation occurs quickly with the solids increasing from 40 to 85 percent at stage II. After that, the remaining water and coalescent leaves by the much slower diffusion process. Film Formation – Temperature and Relative Humidity Effects Studying the previous diagrams, it is apparent that a sealer that forms a continuous film is what is necessary to prevent efflorescence. Any sealer that does not com¬ plete the steps of film formation ends up with an unformed film that can allow the migration of water that results in efflorescence. While the choice of coalescents Connaughton, Rokowski and Ennis—31 does have some control over the for¬ mation of a contin¬ uous film, it is tem¬ perature and rela¬ tive humidity that have the biggest impact on film for¬ mation. Since the vapor pressure of water and coales¬ cent is directly dependent on ambi¬ ent temperature, it makes sense that raising the tempera¬ ture will speed up film formation. However, in the case of relative humidity it is only the evaporation of water, which is FILM FORMATION mrMizsi noutis Figure 6 LATEX FILM FORMATION Ppfymer Latex Watef eraperation. particle concentration. QaccnUtloa. Close -contact particle array, water Jilted TnltUl coalescence” • evaporation of interotltUl water, particle riefarmaHoa Turtber gradual coalescence” – polymer Interdiffoaloc Smooth, {setmpte JUm Figure 7 inversely dependent of relative humidity, which affects film formation. Therefore, as relative humidity is increased, the evaporation of water, and consequently, film formation, is decreased (Figure 9). In this instance, the ratio of coalescent to water is so low that film formation is not allowed to proceed past stage II (from Figure 7 and S) and the result is poor film formation. Only if the temperature is increased, thereby speeding up the evaporation of water and the forma- THREE STEP PROCESS . OF LATEX FILM DRYING Figure 8 tion of the film, can an impermeable, con¬ tinuous film be achieved. Conversely, at ambient tempera¬ ture and low relative humidity, the solvent/ water ratio increases rapidly cre¬ ating a coalescent liquid phase that is favorable to good film formation. At low relative humidi¬ ties, only a drop in temperature to the point where the mini¬ mum film formation temperature is reached, will cause the formation of the THE EFFECT OF RELATIVE HUMIDITY ON THE EVAPORATION OF WATER FROM A LATEX PAINT FILM Figure 9 Connaughton, Rokowski and Ennis—32 film be compromised. Figure 10 shows the change in the relative solvent/water ratio of a drying latex film at three different relative humidities. Under varying conditions, it is essential to match the proper acrylic latex polymer with a coalescing solvent appropriate for the conditions under which the latex will be applied to maximize the probability of success. THE RELATIVE CHANGE IN THE COALESCENT/WATER RATIO IN A LATEX PAINT FILM DRIED AT DIFFERENT RELATIVE HUMIDITIES Figure 10 Application Methods and Conditions for Use of Acrylic Sealers Formulated acrylic sealers are generally applied by some form of spray equipment, whether it is done initial¬ ly in the factory or many years later as a refurbishing application. Conventional, airless, air-assisted airless, or HVLP (High Volume Low Pressure) have all been found to work well in covering the variable shapes of concrete substrates with a film of uniform thickness. It is up to the manufacturer to decide which process best fits the com¬ pany’s production line and conditions. Sealers can be applied 1) on the wet side, which is sealing concrete before it has been allowed to cure, whether it be in a drying chamber or naturally; or 2) on the dry side, on concrete that has undergone some degree of curing. Dry-side sealing is currently the most com¬ mon, resulting in higher gloss potential and lower add-on rates. Each method has its advantages and disadvantages and it is often the geographical location of the plant and the prevailing weather conditions that determine which method to employ. Sealing on Wet Concrete Advantages Disadvantages Deeper, more uniform color post cure Low, variable gloss Ease of handling dry substrate post cure Sensitive to cure conditions Sealing on Dry Concrete Advantages Disadvantages High gloss potential Wet sealer can pick up foreign particles Good protection at Handling and packaging lower add on rates of wet substrate As expected, applying a sealer with an adequate, uni¬ form film thickness is necessary in order to achieve max¬ imum sealer performance. Dry film thickness of one-half mil on dry concrete and one mil on wet concrete are min¬ imum requirements. The spray equipment should be positioned such that equal volumes of sealer are evenly distributed across the substrate and are applied with min¬ imal over spray. It is also necessary for the sealers to be formulated with the proper type and level of coalescent to assure good film formation at the lowest temperatures prevalent in the plant. In areas of high relative humidity, it is recommended that heat is available on the line to assist in the film formation process. Finally, it is very important that stacking and packaging of the substrates is done properly to avoid potential problems. With wet con¬ crete, it is important that the substrates are not stacked too close together prior to film formation, whether in a curing chamber or if being air dried, in order to avoid contamination from the condensate. On dry concrete sealing, it is recommended that the substrates be pack¬ aged in such a way as to allow for venting of the residual water found in the sealer. Removing Efflorescence From Concrete Substrates Once efflorescence has been detected on a substrate, the best method of removal is one involving dry tech¬ niques rather than one with water. This helps prevent the loose particles from being washed back into the capillar¬ ies or voids causing further contamination of the field. Such techniques often employ brushing, vacuuming, high-pressure air, and even light sand blasting. If dry methods do not work, it may be necessary to use a 5 to Connaughton, Rokowski and Ennis—33 10% solution of muriatic acid. Acetic and phosphoric acid solutions have also been used effectively. It is best to test small areas when using any chemical solution to make sure that they will not adversely affect the color or create other surface problems through the reaction of minerals found in the concrete. It is also necessary to wear the appropriate personal protective equipment when handling these types of materials because of the eye, skin, and breathing hazards they present. The runoff from chemical cleaning may also present problems depending on the type of gutters and downspouts that are on the structure and also for the vegetation that is growing nearby. When using chemical cleaning methods, first wet a small area of the substrate field, apply the acid solution, then wait a few minutes before scouring with a nonmetallic brush. Finally, flush with low-pressure water, directing the effluent off and away from the field. Be aware that when using a chemical solution for efflores¬ cence removal, it may be necessary to treat the entire substrate field in order to prevent variations in color of the substrate. OTHER COMMON CAUSES OF WHITENING ON CONCRETE SUBSTRATES Water Whitening Sometimes called blushing, water whitening is caused by the sealer film absorbing water, thereby chang¬ ing the refractive index of the film. This is accomplished by the collection of water in the latex particle interstices, which are rich in hydrophilic components such as salts, surfactants, and acid-rich oligomers. This creates a dif¬ ference between the refractive index of the interstices and the latex polymer particles, which produces the white color. However, once the water is removed, the sealer dries and recovers to a clear film. The occurrence of water and the degree of whiteness is dependent on many factors. The major ones are the amount of rainwater, the amount of standing water against the tile, the duration of the water, the drying time to remove the coalescents, and the film thickness. Geographical location and prevailing weather often con¬ tribute to water whitening, making diagnosis on a caseby- case basis necessary. For example, a slight rain fol¬ lowed by a hot, sunny day may cause no noticeable water whitening. However, a slight rain or dew, followed by a cloudy, humid day, could cause whitening. The one positive about water whitening is that it will completely recover with the removal of water and after a drying period. This usually occurs over a matter of min¬ utes and does not compromise the integrity of the film as long as the sealer was applied at the correct film thick¬ ness and dried properly. The overall disappearance of water whitening is dependent on the weather conditions to which the sealer is exposed. Usually the intensity will begin to fade in a matter of days as each occurrence works to remove the water soluble components and resid¬ ual coalescents from the film. Generally, water whitening should rectify itself with¬ in 30 days; however, this may be extended, depending on the choice of coalescents and the film thickness of the applied sealer. For this reason it is recommended that water whitening be allowed to be resolved with natural weathering. Some manufacturers and contractors recom¬ mend that solvents be used to re-emulsify the sealer, thereby releasing the trapped moisture; however, getting the sealer to reform a film with the same initial integrity can be tricky, particularly if the substrates are in difficult or precarious locations, such as concrete roof tiles on a steep-sloped roof. There are also the hazards associated with solvent use in residential or commercial locations. It is much easier and safer to let water whitening dissipate over time in order to achieve the desired result. Sealer Cracking Sealer cracking is a general term used to describe sealer that did not form a complete and continuous film for any number of reasons. This may include but is not limited to poor coalescent choice or level, poor choice of sealer additives or levels, heavy or low sealer add-on lev¬ els, high solids level if sealing on dry concrete, poor sur¬ face preparation, uneven spray application, poor curing conditions, and poor cement mix. Sealer failure is gener¬ ally whiter than efflorescence and is often misdiagnosed unless the condition is observed closely, preferably under magnification. Using a microscope, sealer failure can be seen as cracking (star cracking) and flaking of the dried film, which bends the light that would otherwise pass through the clear sealer, thereby creating a white appear¬ ance. Often there are broken patterns and breaches in the sealer coating. If sealer failure occurs early enough in the sealing and curing process of the concrete substrate, it can lead to efflorescence, which then shifts blame to the sealer rather than the application. As mentioned earlier, the best method used to detect sealer failure is under a microscope, whether it be in a lab or by using small, inexpensive, hand-held magnifiers. It may also be possible to scrape the surface of the sub¬ strate with a hard object to get the sealer coat to delami¬ nate; however, the flaking is not always visible with the naked eye. Once failure has been observed, the next step Connaughton, Rokowski and Ennis— 34 is to remove all remaining sealer, as further degradation will surely occur over time. This is often achieved by any one of the dry methods mentioned earlier in the efflores¬ cence section, or through more robust means such as highpressure washing. These methods are usually not 100% effective as the sealer rarely comes off completely in this mode of failure. If natural weathering is not an option (natural weathering may takes months to years to resolve), it may be necessary as a last resort to apply a solvent to re-emulsify the remaining sealer. It is also possible to use a dilute solution polymer to not only re-emulsify the damaged sealer, but to also apply additional protection to the substrate. Once it has been determined that a viable surface or coat has been achieved, a second coat using either water or solvent based sealer can be applied. There are, however, a few instances where this restoration procedure may not be appropriate. One of these is in the case of laitance. Laitance is the most com¬ mon surface imperfection found on concrete resulting from incompletely hydrated cement scum, fines, and impurities which are brought to the surface of over wet concrete by bleeding water. This residue forms a weak and fragile layer with little or no bond strength to the surface. Therefore, any sealer applied to concrete with this condition will experience cracking and flaking of the sealer coat. Over time, laitance will weather and begin to clean itself from the substrate; however, mechanical cleaning in the form of light sand blasting or abrasive scrubbing is often used to accelerate the process. Most often it is recommended that the damage substrates be replaced to eliminate the risk of further problems. SUMMARY Efflorescence is a naturally occurring phenomenon resulting when calcium hydroxide, the by-product of cement hydration, is earned to the surface of cement based products by migrating water, allowing it to further react with atmospheric carbon dioxide to form unsightly and aesthetically displeasing, white surface deposits. Acrylic sealers have been found to effectively prevent and control efflorescence using both solvent and water based polymers. However water based acrylic polymers have become more popular because of the ease of use and cleanup as well as the reduced hazards associated with using water based over solvent based products. When properly formulated, applied and cured, acrylic sealers are able to deliver long term, durable protection to a wide variety of concrete substrates. REFERENCES “An Explanation of the Elimination of Efflorescence on Concrete Pavers,” Technical Data, Urban Stone. Concrete Masonry Structures, Cleaning and Maintenance, Concrete Masonry Association of Australia. Matt, R., “The Use of Acrylic Sealers to Prevent Efflorescence On Concrete,” Presentation to NTRMA, January 1997. “Removing Efflorescence From Concrete,” Historic Preservation Technical Procedure 03710-16, U.S. General Services Administration. Slaton, D. and Backus, L., “Efflorescence,” The Construction Specifier, January, 2001. “The Origins of Efflorescence in Concrete and Concrete Products,” QCL Group Technical Notes, March 1999. Improved Weather Resistance of Modified x Bitumen Roof Products Using Reflective Acrylic Elastomeric Roof Coatings INTRODUCTION Low slope roofing involves a different variety of sub¬ strates not used in steep slope applications and certainly different from concrete roof tiles. A large proportion of low slope roofing substrates involve asphaltic type mate¬ rials. Asphalt is an ancient material and has been around for centuries, a residue of the crude oil distillation process. Asphalt is predominant in hot-mop asphalt for built-up roofing. But there are many variations in asphaltic products for roofing and these include, at least in part: asphalt cutback (solvent plus asphalt), flood coat (hot asphalt), asphalt emulsion (asphalt plus water), alu¬ minized asphalt, cap-sheet (granulated mod-bit), and the increasingly popular, smooth surface modified-bitumen. How frequently used is mod-bit and why is it so impor¬ tant to the roofing industry? Connaughton, Rokowski and Ennis—35 Figure 11 – NRCA Low Slope Substrate Market by Square Foot From the pie chart (Figure 11), one can see that for the year 2000, figures from the National Roof Contract¬ or’s Association, based on square footage, show that the fraction of low slope roofing substrates composed of modified bitumen is over 24% ! Several decades ago, the fraction of asphalt built-up roof was over 80%, including some coal tar types of roofing. One can also observe the increase in use of EPDM rubber membranes as well as other single ply membranes, such as TPO, PVC, and Hypalon. Because mod-bit is now such an large portion of roofing substrates, it is helpful to understand the com¬ position, failure method, and opportunities to lengthen the life of modified bitumen roofs. What is modified bitumen? Modified bitumen is a roll roofing product increas¬ ingly used in commercial low-slope roofing applications. From a practical aspect, modified bitumen has advan¬ tages over standard asphaltic built-up roofs due to ease of application, lower labor/installation costs, and reduced likelihood of fire in the absence of hot asphalt kettles. In addition, the “modification” of modified bitumen is done with relatively low levels of a polymer, usually about 15- 25%. There are two types of polymers used for modifica¬ tion. One is APP, which is atactic polypropylene, a very plastic-like material. The other is SBS, (styrene-butadiene- styrene) copolymer, which is very rubber-like. The polymer modification increases the performance proper¬ ties for weathering, durability, and toughness. All of these roll goods have an internal fiberglass or polyester scrim for added tensile strength and puncture resistance. The APP type membranes are generally a little stiffer than the SBS types. Why? Mostly because APP is a stiff, plastic-like polymer, which translates into stiffer mem¬ brane. SBS has a more flexible, rubber-like component in the butadiene content – the “B” component of SBS. As a result, SBS membranes are generally softer, more flexi¬ ble, and can be installed with greater ease, especially in lower temperature climates or in late fall or early spring. The asphalt component that is used in mod-bit is typ¬ ically processed minimally (usually to Type 3 asphalt) by blowing oxygen into the mixture, thus oxidatively increasing MW and increasing melt temperature. During the melt process, the APP or SBS polymeric modifiers are added and mixed homogeneously into the asphalt. In addition to the asphalt, polymer modifier, and scrim com¬ ponents, inorganic fillers are used to add bulk and some¬ times fire retardancy to the sheet. Other processing agents used within the asphalt mixture itself include the use of low molecular-weight asphaltic compatible oils and sometimes waxes which aid in the flexibilization of the mod bit. These oils tend to be very asphaltic-compat¬ ible, but can migrate out of the film along with other asphaltic fractions and cause staining or prevent efficient adhesion of an aqueous, elastomeric roof coating. The migration of oils is more prevalent in APP mod-bit than in SBS mod-bit. Why? These oils are less soluble in the polymer component of APP mod-bit, so they have a greater tendency to migrate out of the membrane. With SBS mod-bit, the oils are much more soluble in the SBS polymer component and take longer to migrate out. Importantly, at the end of the roll-forming process, materials are applied to the mod-bit surface to allow smooth roll/unroll of the mod-bit membrane and to pre¬ vent the formation of a giant “log” of asphalt. Typically, these processing aids are release agents of some type and are usually 1) a surfactant, 2) a powdery, inorganic exten¬ der pigment such as mica or talc, or 3) a solid particulate extender material such as sand. These release agents and the drying oils are what cause the most problem for coat¬ ings on top of mod-bit. Asphalt failure mechanism To understand the ability of acrylic elastomeric roof coatings to increase the life of asphaltic type roof prod¬ ucts, the mechanism of asphalt degradation must first be understood. Asphalt is a complex mixture of literally hundreds of aliphatic and aromatic hydrocarbons. As it weathers, it breaks down chemically by absorbing UV radiation from the sun. The UV energy causes molecular vibrations and debonding, ultimately observed as crack¬ ing and chalking of the asphalt. Heat greatly increases the reaction rates and is one of the main causes for asphalt degradation. An additional weathering component is the exudation of low molecular weight fractions or oils from the asphalt which act as plasticizers to improve the flexibility Connaughton, Rokowski and Ennis—36 Figure 12- Exterior weathered APP modified bitumen, five years exposure. of the roofing material. Because they are water soluble, this exudate can be seen on recently installed mod-bit roofs where ponding is evident. The readily observed, black-brown residue is typically called “tobacco stain¬ ing.” Its loss further contributes to degradation by reduc¬ ing the long-term flexibility of the mod-bit membrane. Heat greatly increases the migration and loss of oil from the mod-bit. Figure 14 – 300 X magnification of uncoated, weathered BUR surface Asphaltic analysis: unweathered, weathered, and coated To understand the effects of weathering of asphaltic materials, Rohm and Haas has done chemical analysis on exposed unexposed and coated asphaltic built-up sys¬ tems. To set up the experiment, samples of built-up roof were exposed outside, uncoated as well as coated with elastomeric roof coating. An identical but unexposed sample was kept in the lab. After six years’ exposure, the samples were removed and analysis was performed on the unexposed, weathered/uncoated and the weathered/ coated material. Figure 13 — Asphaltic built-up roof analysis. Figure 15 – 300 X magnification of coated, weathered BUR surface The components of the asphalt before and after weathering were separated using both filtration and sol¬ vent extraction and weighed and compared to the original weights of samples. Asphalt fractions are soluble in dif¬ ferent solvents. Asphalt components soluble in aliphatic hydrocarbons such as heptane are called maltenes. Asphalt components soluble in aromatic solvents like THF (i.e. tetrahydrofuran) are called asphaltenes. Neither solvent affected the acrylic coating. Weight loss measurements show that UV degradation reduces the weight of the hexane-soluble or low MW plas¬ ticizing component, which we would expect to cause embrittlement and cracking. Analysis of the acrylic coated material shows that all weathering involves some loss of heptane solubles. The THFsoluble component was much higher for the uncoated sample than the coated sample. Scrim weight and inorganic filler content remains simi¬ lar. The big difference comes in protection of the asphaltic component from UV. Microscopy pho¬ tographs (Figures 14 and 15) show that uncoated/ weathered asphaltics have craters and signifi- Parameter Uncoated/ Unweathered Uncoated/ Weathered Acrylic Coated/ Weathered Initial Weight in Grams/2.5cm2 2.79 2.84 3.51 Grams Heptane Soluble 1.24 1.10 1.19 Grams THF Soluble 1.16 1.62 1.17 Grams Mat 0.24 0.23 0.24 Grams Fines 0.28 0.28 0.26 Grams/2.5cm2 Recovered 2.40 2.72 3.43 Wt % Heptane Soluble in Asphalt 51.7 40.8 51.2 Connaughton, Rokowski and Ennis—37 cant surface pitting. When the acrylic coating is carefully peeled away from the coated, exposed BUR sample, the asphalt surface appears smooth with some surface crack¬ ing, but without craters and pitting. Mechanism of Coating Protection Acrylic coatings’ ability to protect the asphaltic sub¬ strate and extend the service life of the roof is based on two mechanisms. The first is that the acrylic polymer is not susceptible to UV damage because the polymer is transparent to UV, unlike with aromatic components of asphalt. Since the acrylic polymer is not UV damaged, it is very durable on the roof when exposed to sunlight. Clearly, if the polymer is UV transparent, something in the coating must be UV blocking to the substrate in order for the coating to protect the asphalt. The coatings for¬ mulation is composed of materials that DO block UV from penetrating to the substrate. These ingredients are titanium dioxide and zinc oxide, the two most important hiding and UV blocking pigments in the formulation. The second is that the acrylic coating provides a water resistant barrier over asphaltic surfaces. Acrylic coatings are able to resist penetration of bulk liquid water but allow the permeation of water vapors, hence are called “breathers.” One benefit of the Acrylic roof coatings have been around for quite some time and have primarily been used over SPF, metal, aged BUR, or mod-bit. So what is different about the acrylic coating for fresh asphaltics? In order for the acrylic coating to be hydrophobic enough to coat and adhere to fresh asphaltics, the polymer needed to be modified to that it was more “asphalt-like.” This means that one of the components of the polymer had to be very hydrophobic or oil loving, like asphalt. This was achieved through a monomer that has a string of 12-14 carbons on it versus the normal one-carbon-side chain, making it very oil-like. Because of this, it is quite com¬ patible with the substrate and also very water resistant. In fact, many of the coatings based on this polymer have very low permeability values (i.e., 1 to 4 perms) as com¬ pared to 20-35 perms for a typical acrylic roof coating. This helps improve then adhesion by preventing water from reaching the asphaltic/coating interface. Figure 16 demonstrates the long-term durability of white acrylic roof coating versus aluminized asphalt. The acrylic coating remains white and thus cooler and pre¬ vents the migration of asphalt stains from the substrate. Unweathered Weathered water resistance is to minimize solubilization of the low MW asphaltic components and subsequent migration out of the mod-bit and into the coating. In addition, when the pigmented white coating is applied over black asphaltic materials, the surface temperature is significantly lowered by up to 50o F (28o C). The lowering of temperature will further reduce chemical reactions, asphalt degradation, and extend the life of the roof. Rohm and Haas often does lab and field work to understand the failure mechanism of coatings applied to asphaltic type roofing products. The mod-bit release agents have a tendency to inhibit the adhesion of waterborne coatings, as does the composition of the asphalt and modifier (APP and SBS). Asphalt is a very hydrophobic and oily sub¬ strate to begin with and is difficult to “wet-out” with a hydrophilic (water loving) roof coating. In addition, the surfactant that adheres to the mod-bit surface and is used to prevent sticking of the roll can also prevent the coating from getting a good “bite” to the mod-bit. This simply means that the coating is trying to adhere to an oily surface (i.e. mod-bit) but before it can do so, must try to adhere to a slippery surfactant layer on smooth mod-bit Figure 16- Acrylic vs. aluminized asphalt on fresh APP modified bitumen: unweathered vs. weathered. Connaughton, Rokowski and Ennis— 38 Blister Resistance of Acrylic Coatings over Asphalt Emulsion Adhesion of coatings in ponded situa¬ tions is sometimes a challenge for acrylic roof coatings. Asphalt emulsion is difficult to adhere to because of the variation in asphalt grade and emulsifiers. To simulate ponding water conditions, three waterborne coatings were applied to a one-week-cured asphalt emulsion, dried two weeks, and immersed continuously in water for 12 days. Results in Figure 17 show that, indeed, the proprietary acrylic exhibits no blistering, while two other commercial products blister. Energy Savings Through White Roof Coatings Figure 17 -Asphalt emulsion coated with acrylic roof coating, 12-day water immersion. Finally, the solar reflectivity and emissiv¬ ity of acrylic roof coatings versus aluminized asphalt were measured according to EPA’s Energy Star® protocols. From the table below, it is clear that the acrylic roof coating has higher solar reflectivity and emissivity than the aluminized asphalt coating. Coating Solar Reflectivity Emissivity Proprietary Acrylic 84% 91% Aluminized Asphalt 55% 53% Target 65% minimum initial >80% REFERENCES: Anonymous, “Aluminum Coating Applications,” Contractors Guide, March 2000. Barth, Edwin J., Asphalt: Science and Technology. “The Effects of Acrylic Maintenance Coating on Reducing Weathering Deterioration of Asphaltic Roofing Materials,” ASTM Symposium on Roofing Research and Standards Development, Montreal Canada, June 1994. Lorenzo, Gilbert, Twenty Five Years ’ Experience in SBS Membranes. SUMMARY Asphalt roofing substrates fail by loss of low molec¬ ular weight fractions, heat-induced degradation, and frac¬ turing. Aluminized asphalt coatings and acrylic elas¬ tomeric roof coatings have been used over modified bitu¬ men roofs but sometimes fail in the areas of asphalt stain bleed through or blistering in ponded water situations. In addition, aluminized asphalt coatings do not generally pass emissivity values recommended by the EPA. “MRCA/NRCA Research Report: APP and SBS Modified Bitumen Roofs: A Survey of Field Performance,” Midwest Roofing Contractors Association and National Roofing Contractors Association, March 1996. O’Dea, Teresa, “Cool Alternatives to Hot Roofing,’ RSI, September 2001 “Water Soluble Residue from Asphalt Roofing Products (‘Tobacco-Juicing’),” Technical Bulletin/Asphalt Roofing Manufacturers Association, November 1994. Connaughton, Rokowski and Ennis—39 Elastomeric Acrylic Roof Coatings for EPDM Roofing INTRODUCTION: The EPDM market presently holds about thirty to thirty-five percent of the 3.6 billion ft2 low slope roofing market, growing rapidly at the expense of other conven¬ tional roofing systems. Black EPDM membranes are the predominant choice for new low slope roof construction. The Environmental Protection Agency (EPA) has become a major influence in the roofing market. The EPA EnergyStar1′ program promotes manufacturing and marketing of energy-efficient products, thereby potential¬ ly reducing combustion-related air pollution (green house gases and NOX) from electrical power generation. The EnergyStarR program has issued requirements for reflec¬ tivity for low slope roofs, both as new installations and after three years. The requirements are: the membrane must have 65% initial reflectivity and 50% reflectivity after three years exposure. Black EPDM membranes do not meet these reflectivity requirements set by the EPA for an EnergyStarR rating. However, a properly formulat¬ ed white-coated black EPDM membrane roof will meet the EnergyStarR requirements. Uncoated EPDM membranes, because they are black, absorb significant solar heat energy. The high heat energy influences the chemistry of the EPDM membrane and leads accelerated weathering of the EPDM mem¬ brane. Applying a white elastomeric roof coating to the black EPDM also reduces the heat load on the EPDM and increases its service life. Rohm and Haas Company has been working cooper¬ atively with EPDM and coating manufacturers to develop new technology to allow the acrylic elastomeric roof coating to have excellent bonding to the black EPDM membrane. This will allow the EPDM to meet EnergyStar1′ requirements and enhance the service life of an already durable membrane. New EPDM Membrane Chemistry New EPDM membranes contain additives that improve processing and field performance and also release agents to ensure that the membrane will not adhere to itself during vulcanization. These required additives sometimes interfere with the adhesion of an elastomeric roof coating. Elemental surface analysis done on a commercial EPDM membrane showed the following inorganic elements and organic moieties on the surface of the EPDM membrane. Element % of Surface C-C 50.56 COR 3.76 COOR 1.80 O 18.03 N 1.13 Zn 15.71 Na 0.00 Ca 0.00 S 7.12 Sil .89 Al 0.00 On the surface of the EPDM membrane, there are two inorganic elements that interfere with adhesion of an acrylic roof coating. These are: zinc (Zn) and silicon (Si) The surface of the EPDM can be treated with new technology products. After treatment, the EPDM surface is more easily bonded to the roof coating because a large portion of the zinc and silicon-containing components are significantly lowered, as shown in this table: Element % of Surface C-C 64.50 COR 5.22 COOR 2.89 O 14.92 N 0.00 Zn 3.14 Na 1.37 Ca 6.02 S 1.94 Si 0.00 Al 0.00 Connaughton, Rokowski and Ennis—40 Figure 18 (left) – No pretreat¬ ment. Figure 19 (right) – Pretreated. Coating Adhesion Testing Since EPDM membranes are chemically inert, it is difficult to get adequate adhesion with them. New technology can allow an elastomeric acrylic roof coating to have excel¬ lent adhesion to the membrane, even in areas where there is water ponding. A nylon mesh was imbed¬ ded in the membrane during appli¬ cation. Photos 18 and 19 illustrate a 1 O-year-old EPDM membrane coat¬ ed with an elastomeric acrylic roof coating and a nylon mesh imbedded in the membrane during application being pulled off after one year of weathering. Microphotographs of Treated and Untreated New EPDM Membrane The microphotographs (Figures 20 and 21) clearly show the differ¬ ence in crystalline surface changes of the untreated and treated EPDM membrane. Adhesion of Acrylic Roof Coatings to EPDM Adhesion of the elastomeric acrylic roof coating to the EPDM membrane will eliminate blistering in areas where there is ponding water. Figures 20 and 21: Microphotographs of treated and untreated new EPDM membranes. Connaughton, Rokowski and Ennis— 41 Figure 22: Peel Adhesion of Manufacturers A and B EPDM with and without treatment graph. changes in ten¬ sile strength and elongation properties of the rubber membrane. Generally as the processing oils leech out, the membrane becomes less elastic but stronger. This leeching phe¬ nomenon is evidenced by loss of mem¬ brane thickness The graph shown in Figure 22 illustrates the improvement in adhesion of both a new and aged EPDM membrane that was cleaned with a new technology prod¬ uct. Improving EPDM Solar Reflectance Using Coatings: EPA’s EnergyStar® program is having a major influ¬ ence in the roofing market. Legislation in certain states (i.e. California, Georgia, Texas, and even in Illinois), and some local building codes are making it mandatory that low slope roofs have an initial and aged solar reflectance that meets the criteria for EnergyStar®. The initial solar reflectance for a new roof must have a solar reflectance minimum of 65%. A black EPDM membrane have a solar reflectance of -10% reflectance, which is below the requirements of EnergyStar®. Applying a white elas¬ tomeric acrylic roof coating will have an initial solar reflectance of 80% reflectance or higher, which is well above the criteria for initial reflectance as required by EPA. Table 23 illustrates some reflectivity measurements taken on several reflective coated projects through out the country. and dimensional shrinkage. Even though the elongation modulus and increases tensile strength, generally make the EPDM less flexible. Puncture resistance and tear resistance will be reduced as the EPDM membrane loses its elasticity. Figure 24 shows a 15-year EPDM mem¬ brane with a severe tear which was caused by the mem¬ brane shrinkage over time. Ballasted EPDM membranes will also lose elonga¬ tion even though the direct UV rays from the sun are somewhat screened. The radiant heat IR portion of the sun will still heat the membrane to temperatures in excess of 150 degree Fahrenheit. The high membrane temperatures “bake” the processing oils out and the membrane loses some of its initial elasticity. A paper was presented at the 11th Conference on Roofing Technology by Paroli, DelGado, Dutt, Smith and Simmons on “Shrinkage of EPDM Roof Membranes: Phenomenon, Causes, Prevention and Remediation.” In this paper, stud¬ ies on elongation properties showed that test samples taken from the top layers of the membrane had signifi¬ cant loss in elongation properties compared to the bottom sample of the same membrane. Weathering Properties of EPDM and the Effects of Coatings: Weathering of the EPDM rubber membranes can cause The elastomeric acrylic roof coating is a UV-resistance, watertight monolithic membrane. In essence, it is a membrane fully adhered to another membrane. The coat¬ ing reflects the UV rays and will keep the rubber membrane temperature at approxi¬ mately 10 degrees higher than the ambient temperature instead of the greater than 150 degrees Fahrenheit if left uncoated. At these lower temperatures, the oils in the EPDM will remain intact, the thermal Figure 23 – Reflectivity measurements on reflective coated projects. modulus of the rubber with not increase, Energy Star Specification Performance Specification Roof A 3 years Roof B 4 years Roof C 5 years Initial Solar Reflectance Maintenance of Solar Reflectance > 65% > 50% 83.4% 69.2% 83.0% 66.9% 77.0% 57.8% Connaughton, Rokowski and Ennis— 42 and the tensile strength of the membrane will not increase as much so that the EPDM membrane will still remain more flexible. All this will result in an even longer life and lower life cycle cost for the EPDM roof. CONCLUSION: Elastomeric acrylic roof coatings have been used commercially on EPDM roofs since the mid 1980s. Recent improve¬ ments in adhesion technology and the need for enhanced solar reflectance have created an even greater opportunity for the combination of these two highly durable construction materials in low slope roofing. REFERENCES: 1 Dupuis, R.M. and Haas, A.K., “Temperature-Induced Behavior of New and Aged Roof Membranes,” Proceedings of the Second International Symposium on Roof and Roofing, 1981, Volume 1, pp 66. Paroli, Ralph M, Delgado, Ana H, Dutt, OM, Smith, 24 – A 15-year EPDM membrane with a severe tear caused by membrane shrinkage over time. EPDM Roof Membranes: Phenomenon, Causes, Prevention and Remediation,” 1995 pp 90-110. R.T. Vanderbilt Company, Inc; The Vanderbilt Rubber Hand Book, thirteenth edition, 1990. Thomas L and Simmons, Terrance; “Shrinkage of Connaughton, Rokowski and Ennis—43
