By Girish C. Dubey, President STAR, INC.
Rheology is the property that deals with the flow behavior of a coating during the manufacturing and application process. The very basic requirement for a coating to perform adequately is to flow evenly and cover the substrate evenly. This requirement is, however, different for different products. For example, a house paint should be thick enough to be loaded sufficiently on a brush or roller yet have the ability to thin down and flow evenly, when the brush or roller is pushed on the surface. If the paint is too thin or thick the application will be poor and performance unacceptable. Similarly, Sealcoating products must have proper rheology to function as pavement coatings. They shall be thick enough to hold sand/aggregates in suspension, yet fluid enough to flow into the pavement profile and covering squeegee or brush marks. This is the essence of Rheology.
The use of rheological additives in sealcoating formulations is essential. They are known by different names; thickeners, rheology modifiers, bodying agents etc. In Sealcoatings, several rheological additives are used which vary in their effectiveness for rheology modification as well as their effect on the overall performance of the cured sealcoating. Let us review these additives in the light of their performance as rheology control agents, strengths and weaknesses.
Chemical compounds e.g. sodium sulfate, magnesium chloride, acetic acid etc. have been used in sealcoatings where they have been observed to produce instantaneous thickening. The thickening effect is derived from the fact that these chemicals ionize in water to produce acidity which in turn, has a flocculating effect on the clay in the sealcoating formulation. In simpler words, the clay particles flock together when their respective electrical changes (negative) are robbed by the positive charges, arising from the ionization of the salts. And that is seen as thickening.
Although such materials are relatively inexpensive and quite efficient as thickener, they introduce permanent water sensitivity in cured sealcoating films. In other words, salt additives stay in the sealcoatings film and re-absorb water at rain events or under high ambient humidity. The water thus re-absorbed swells the sealcoating film which, in turn, is torn by vehicular or pedestrian traffic. The premature failure of sealcoatings have been attributed to poor water resistance in numerous situations.
The absorbed water is also capable to re-emulsify the cured film under the grinding action of the traffic. This is commonly seen as “brown colored” water puddles. The process of “re-emulsification” is the primary cause of the tracking problem where the re-emulsified sealer is likely to tracked into businesses or homes. The use of these chemicals should, therefore, be avoided.
2. NATURAL GUMS
Xanthan Gum is used quite frequently, particularly for making pails products for the retail markets. The retail products are pre-blended version of sealcoating materials where water and aggregates are added at the time of manufacturing and the final product is packaged in 5-gallon pails. Since the product are designed as ready-to-use, the major requirement is no or very soft settling of the contents in the pail. Furthermore, the contents in the pails should be easy to re-disperse to usable consistency, as the homeowner is not expected to be equipped with all kinds of stirring gadgets.
Gums do a fair job in imparting the right viscosity, through the formation of gels within the sealcoating. They do not affect the dispersion of clay and fillers in the sealcoating, therefore, have lesser or no tendency to destabilize the sealcoating system.
Water sensitivity of gums is again the major problem because they stay in the cured film of sealer and re-absorb water when it rains, or the humidity becomes too high. The clays in the sealer, again, swell as a result of water re-absorption, rendering the sealcoating weak and vulnerable to traffic. Severe tracking problems have been reported in coatings using natural gums as rheological additives. Cost effectiveness is also poor.
3. BENTONITE CLAYS
These specialty clays have been commonly used since the inception of sealcoatings formulations rheology control.
Bentonite is generally used in conjunction with ball clays in sealer formulations. Bentonitic clays act as primary dispersants for tar or asphalt in the water phase along with the ball clays. They have the exceptional ability to form a firm (but false) body in the sealcoating material which breaks down easily under the pressure of the brush, squeegee or spray, rendering a good flow and leveling that are essential for even coverage. The sealcoating material stops flowing soon after the removals of the pressure from the application tool. This phenomenon in coatings flow behavior is called ‘Thixotropy’; where the viscosity drops on shearing action of brush, squeegee or spray and reconstitutes when the shearing actions is removed. Bentonitic clays impart real thixotropy to sealcoating formulations and their use is quite frequent. Despite of their water sensitivity, bentonitic clays are essential for good rheological control. The water sensitivity of the sealcoating can be minimized with judicious choice of ball clays and specialty chemicals in the sealcoating formulations. The cost effectiveness is good.
4. POLYMERIC THICKENERS
In the last few decades the industry has witnessed the use of a myriad of latex additives which have been claimed to deliver all that a sealcoater could ever dream of; some are true and some false. We will confine our discussion to their use as rheological additives. The latex chosen as rheological additive must not have a destabilizing effect on the sealcoating material. The sealcoatings frequently are formulated with specialty chemicals, emulsifiers and surfactants. The latex used should never rob the electrical charges from the clay or the bitumen particles, nor clash with the specialty chemicals in the formulation. Some latex additives have been found to weaken the sealcoating film or make them less resistant to chemicals. On the other hand, some select polymeric rheological additives boost the performance in real terms like faster drying, uniform dried color of the sealer, enhanced resistance to petrochemicals, de-icing salts and overall longevity.
The choice for the proper polymeric rheology addictive is the key to superior performance. The formulator or the sealcoating professional is advised to familiarize himself/herself with all the available additives and their specific attributes, prior to making the selection. The cost effectiveness can be excellent with the choice of the right material.