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The modification of bitumens by polymers is a common process, which aims at reinforcing the resistance of bituminous binders according to their applications (road construction or waterproofing), traffic or even the climate.
While some polymers can be dispersed easily by addition and mixing in a tank, elastomers very often require specific, energy-intensive and expensive industrial dispersion devices. The success of such incorporations is based on a few basic principles, including the lab study of the raw materials in order to validate the compatibility between the row binder and the polymer.
Bitumen, or rather bitumens, are composed of a wide variety of molecules that are classified into four main families: asphaltenes (black solid) on the one hand, and maltenes on the other, which include resins , aromatics and saturated oils. These families constitute a molecular continuum and their proportions influence the colloidal equilibrium as well as the rheological properties of the binder. A specific molecular configuration between these families makes it possible to maintain the asphaltenes in suspension, in close connection with the resins, surounded by a fluid matrix composed of the rest of the maltenes. This organization is responsible for the visco-elastic behavior of bitumens, behavior that we seek to optimize by adding plastomeric or elastomeric components in the mix.
The incorporation of a polymer in this system in equilibrium can cause its destabilization. Commonly, the addition of the elastomeric polymer is followed by a so-called “maturation/digestion” stage, during which the polymer will absorb some of the maltenes in the bitumen and “swell”. By “pumping” a fraction of the binder,the polymer often trigger the destabilization of the binder, then leading phase separation during storage and loss of the visco-elastic properties of the binder.
Control in the laboratory
In this context, the initial laboratory study is essential to avoid any failure during the industrial production, which involves significant resources in terms of equipment and materials. During these studies, pilot batches of a few hundred grams are produced by high-energy dispersion of polymer pellets within the binder at high temperature using a special device. The result of the dispersion can then be assessed visually with an epi-fluorescence microscope as described in standard NF EN 13632. This makes it possible to assess the homogeneity of the distribution of the polymer and to study the storage behavior . Once validated, the binder-polymer balance can then be evaluated with regard to its mechanical performance by springback, force-ductility, tensile and other rheological tests.
The most demanding laboratories will try to link the contents of asphaltene and maltenes with the results obtained. To do this, the decomposition of bitumen using the Iatroscan, a simple chromatographic method to implement. The binders with the best compatibility and performance results with polymers are isolated, and allow to build an interneal data base for the company and accelerate the identification of the right candidates for its future modification projects.
The process of incorporating a polymer into bitumen involves mixing two materials with different properties, under favorable temperature conditions, around 180°C. At this temperature, the viscosity of the bitumen is lowered, which facilitates the dispersion of the polymer. The latter, under the effect of heat, softens to gradually melt within the binder, soaking up compatible molecular fractions. During this step, the most common strategy aims to disperse the polymer as finely as possible. Its elastic nature makes this step difficult, hence the use of a device developing a high shear rate. But the grinding of the polymer in the binder, at rates of several tons per hour, involves deploying very high electrical power, reaching 75 kW. In addition to the consequent energy consumption, this process represents a very important investment, both for purchase and maintenance.
Another strategy, less widespread, is possible. It consists in integrating the polymer pellets by coating them with bitumen. The latter passes through a thin film production cell, forming a funnel in the middle of which the polymer granules are added at the appropriate rate, so that each polymer granule is completely coated by the binder, and isolated from the other granules (illustrations on our “PMB production units” page). The suspension of granules in the hot binder is pumped into a tank whose stirring device allows the maintenance of homogeneity. Thus isolated from each other, the polymer granules can no longer form clumps (clumps that would be impossible to disperse without grinding), and can absorb the bituminous fractions necessary for their swelling during a maturation phase.
This process, based on understanding the physico-chemical phenomena that take place during mixing, is the one that VIALAB has chosen to develop. In addition to a reduced investment, it allows for 30% electrical energy savings, compared to traditional solutions involving a high shear rate. This makes it possible to offer an effective technical solution, which is perfectly in line with our committed approach to environmental protection.
Vincent HESRY, Ph.D and Antonin RICHARD, Chemist Engineer.