Building & Construction – New Papers: Paint and Coatings

Asphalt is one of the most widely used materials for road construction. According to the Asphalt Institute, around 95% of all roads in the United States are made of asphalt binders. One of the most important and challenging tasks of the asphalt industry is to develop stronger and more durable pavements which are able to withstand high volume traffic, continuous increase in the traffic flow as well as variations in climate. Therefore, more focus has been put on research in transport related areas in the last several years, including new and improved road materials. At the moment significant research and development efforts in asphalt technologies is taking place worldwide. One of the most developed technologies is the application of modified bitumens. Modified bituminous materials can bring real benefits to highway maintenance/construction in terms of better and longer lasting roads and savings in total road life cost. Typical modifiers include polymers, chemical additives, or a combination of the two.

Polyphosphoric acid (PPA) is a chemical modifier and has been used for this purpose in North America for over thirty years. Typical dosage of PPA ranges between 0.25 wt% and 1.0 wt% (based on the weight of the binder). An estimated 100 to 400 million tons of asphalt mix that has been modified with PPA have been utilized on U.S. highways in the recent years.

In the U.S., PPA is used in various applications which involve preparation of paving or roofing grade bitumens. These applications involve direct modification of bitumen binder with PPA, preparation of PPA-modified binders in combination with SBS polymers, the application of PPA as a catalyst in preparation of bitumen modified with reactive terpolymers, and as an accelerator during air-blowing process. In neat paving bitumen, PPA increases the high-temperature Performance Grade (PG) rating of the bitumen while maintaining the low-temperature properties. Significant improvements in the water‑sensitivity of mixes are also obtained. In polymer-modified bitumen, the use of PPA provides these same benefits and also allows for a significant reduction in the level of polymer required to meet elastic recovery requirements. The mechanism by which PPA interacts with bitumen to improve its rheology and overall properties is still under investigation. One theory that has been put forward suggests that PPA reacts with various organic functional groups in bitumen breaking up asphaltene agglomerates and allowing the individual asphaltene units to form a better dispersion in the maltene phase. The dispersed individual asphaltene units are relatively more effective in forming long-range networks and in turn contribute to elastic behavior, Figure 1.

Figure 1. Possible Mode of Action of Polyphosphoric Acid in Asphalt Modification

Gypsum is one of the oldest known materials and is used in a variety of applications including construction, medicine and agriculture. Small quantities of high-purity gypsum are used in confectionary, food, brewing and pharmaceutical industries. Gypsum is also used in sugar beet refining, as cat litter and as an oil absorbent.

Nowadays, gypsum used in construction is used primarily in the form of building blocks, spray-on coating, or as a main component of construction boards. Production of gypsum boards involves preparation of a calcium sulfate slurry which is fed between continuous layers of paper on a board machine. As the board moves down a conveyor line, the calcium sulfate recrystallizes or rehydrates, reverting to its original rock state. The paper becomes chemically and mechanically bonded to the core, and the board is then cut to length and conveyed through dryers to remove any free moisture.

Condensed phosphates can be used in various stages of the manufacturing process of gypsum boards and other gypsum-derived products. Phosphates like sodium tripolyphosphate (STPP), tetrasodium pyrophosphate (TSPP), sodium acid pyrophosphate (SAPP) and sodium hexametaphosphate (SHMP) act as dispersants for the calcium sulfate slurries and enhance the structural integrity of gypsum boards.

Another important parameter which significantly affects manufacturing process, future performance and integrity of the board is gypsum setting time, which needs to be carefully controlled. Polyphosphates such as sodium trimetaphosphate (STMP) are typically used to extend the setting time. At very low dosage, polyphosphates strongly retard the hydration of calcium sulfate by interactions between polyphosphate and calcium ions.

Condensed phosphates are extensively used as dispersing agents in pigmented water-based systems, primarily paper coating “colors” and water-based paints. Phosphates, such as tetrasodium pyrophosphate (TSPP) and tetrapotassium pyrophosphate (TKPP), aid in the wetting and even dispersion of pigments in formulating the final product. In latex paints, TKPP and potassium tripolyphosphate (KTPP) are the preferred dispersing agents, yielding systems of low and stable viscosities, but also acting as sequestrants.

Dispersing agents are used to assist the mechanical process of dispersing solid, insoluble pigments in water. Sodium polyphosphate and potassium phosphate are used as dispersing agents for emulsion paints, plasters and adhesives. Sodium polyphosphate also improves the storage stability of these materials when applied in combination with polyacrylates and due to its high dispersing effectivity; optimal particle size is rapidly achieved. Sodium polyphosphate is suitable as a dispersing agent for waterborne paints, slows down sedimentation, reduces water hardness and offers a good binding capacity with regard to calcium and heavy metal ions. It also deflocculates pigments and fillers in aqueous suspensions, thereby, high solid contents of emulsion paints with low viscosity can be achieved. Potassium phosphate helps with low and rapid end viscosity of dispersing process and is best suitable as dispersing agent for all aqueous building paints and plasters, caused by the excellent storage stability. It also prevents white shadings and carbonisation of external wall paints.

Ammonium polyphosphates are also efficient flame retardants for application in intumescent paints, coatings, polyolefin, polyurethanes, thermosets and more. These are non-halogenated flame retardants that provide efficient fire protection by generating intumescent systems, lower toxicity of fumes, reduced corrosivity of fumes thus improved preservation of structures and good compatibility with dispersion or polymer.

Phosphates are also used in a wide variety of applications in the coatings industry. Phosphate coatings are a crystalline conversion coating for steel and other metals that is formed on a ferrous metal substrate. The process of phosphate coating is employed for the purpose of pretreatment prior to coating or painting, increasing corrosion protection and improving friction properties of sliding components. In other instances, phosphate coatings are applied to threaded parts and top coated with oil to add anti-galling and rust inhibiting characteristics. The main phosphate coatings include manganese, iron and zinc. Of the numerous phosphate coatings available, manganese phosphate coatings are the hardest, while providing unbeatable corrosion and abrasion protection. In comparison to zinc phosphate coatings, manganese phosphate coatings offer continued wear protection after the breaking in of components that are subject to wearing. Uses for manganese phosphate applications include the production of bearings, bushings, fasteners and other common industrial products. Zinc phosphate coatings are mainly used for rust proofing on ferrous metals. They are applied by immersion or spraying. Zinc phosphate is a lighter alternative to manganese phosphate, while providing resistance to harsh elements that tend to wear products quickly. Iron phosphates are used as a base for further coatings or painting and are applied by immersion or by spraying.

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