پلیمر اصلاح شده (PMB)

Use as needed

While the benefits of using modified asphalts are widely acknowledged, not all asphalt mixes or treatments need to be modified. Each application should be evaluated to determine if the traffic loading, anticipated service life, environmental conditions and desired performance justify the use of modifiers. Modified asphalts can be a good investment.

The rheological properties of conventional binders may be modified by the introduction of:
• Elastomers;
• Plastomers;
• Crumb rubber;
  The modification is costly and is normally justified when bituminous surfacing are subjected to severe conditions such as:
  • Steep gradients;
  • Very high road surface temperature;
  • High traffic loading; or
  • Heavily trafficked intersections. Modification may also be advantageous for surfacing on highly flexible and cracked pavements, where an improvement in the rheological properties of the bitumen is required. Use in such applications should be guided by expert opinion. In addition to the primary aims above, the range of properties improved include
  • Durability;
  • Aggregate retention;
  • Resistance to permanent deformation;
  • Resistance to fatigue cracking;
  • Cohesion (internal strength);
  • Elasticity;
  • Viscosity less susceptible to temperature changes.
  • Modification agents The primary aim of the modification of bitumen for use in structural layers is to increase the resistance of these layers to permanent deformation at high road temperatures without compromising the properties of these layers over the rest of the prevailing temperature range.
    The use of polymer modified bitumen to obtain improved performance is rising as a result of increases in tire pressures, axle loads, and higher traffic volumes.
    Improved performance can be achieved in two ways, both of which are aimed at reducing the permanent strain:
    
  • An increase in the elastic component with an associated reduction in the viscous component; and
  • Stiffening of the bitumen to reduce the total viscoelastic response of the layer.
  • The modification is achieved by the introduction of polymers (including crumb rubber), aliphatic synthetic wax or naturally occurring hydrocarbons.
  • Polymers can be broadly categorized as “elastomers” (sometimes referred to as thermoplastic elastomers) for improving the strength and elastic properties of a binder, and “plastomers” (sometimes referred to as
  • thermoplastic polymers) for increasing the viscosity of the bitumen.

    Process for preparing of PMB polymer modified bitumen

    Composition of polymer modified bitumen is bitumen plus polymer and a compatibilizing agent prepared by reacting one or more unsaturated monomers with a bituminous composition comprising asphaltenes or resins. Asphaltenes and resins are two of four broad component groups found in bitumen (the others are aromatics and saturates; resins, aromatics and saturates are collectively known as maltenes) . Asphaltenes are n- heptane insoluble black or brown amorphous solids, and are generally considered to be highly polar and complex aromatic materials of fairly high molecular weight. Resins are soluble in n-heptane and are typically dark brown in colour, solid or semi-solid and polar. In one embodiment of the invention, the bituminous composition comprising asphaltenes and/or resins is a bituminous composition comprising all four components, i.e. resins, aromatics, saturates and asphaltenes.

    Composition of PMB

    In another embodiment of the invention, the bituminous composition comprising asphaltenes and/or resins is a composition consisting solely of asphaltenes, a composition consisting of solely of asphaltenes and resins, or a composition consisting solely of resins. It is believed that the reaction provides “surfactant” type structures wherein a short polymer chain is chemically bonded to an asphaltene or a resin. It is thought that the surfactant structure consists of a bitumen-like moiety and a polymer-like moiety, and can therefore improve the interaction between the bitumen and the polymer in the polymer-modified bitumen. The polymer-modified bitumen compositions of the invention are essentially physical blends of bitumen, polymer and compatibilising agent wherein the compatibilising agent improves the interaction between the bitumen and the polymer. The compatibilising agent helps to avoid phase separation in the bitumen composition, and allows the use of smaller quantities of polymer to achieve desirable physical properties. Additionally the compatibilising agent may improve the rheological properties of the bitumen composition.

    Process for preparing of PMB polymer modified bitumen

    Process for preparing a PMB polymer modified bitumen binder composition in the substantial absence of cross linking agents by heating a bitumen component in a stirred tank to a temperature from 185° C. to 221° C.; adding a block copolymer composition to the bitumen component while the bitumen component is being stirred to form a homogeneous mixture of the bitumen component and the block copolymer composition; and continuing to stir the homogeneous mixture while maintaining the temperature from 185° C. to 221° C. for a period of time from 4 hours to 30 hours thereby forming a cured polymer modified bitumen binder composition.

    Polymers in PMB modified bitumen

    The block copolymer composition utilized in the process comprises a diblock copolymer comprising one block of a Monovinyl aromatic hydrocarbon and one block of a conjugated diene having a peak molecular weight from 30,000 to 78,000 and a vinyl content from 35 to 80 mol percent based on the number of repeat monomer units in the conjugated diene block, and optionally one or more block copolymers comprising at least two blocks of monovinyl aromatic hydrocarbon and at least one block of conjugated diene, the block copolymer selected from linear triblock copolymers having a peak molecular weight that is 1.5 to 3.0 times the peak molecular weight of the diblock copolymer, multiarm coupled block copolymers having a peak molecular weight that is 1.5 to 9.0 times the peak molecular weight of the diblock copolymer, and mixtures thereof, wherein each block copolymer has a vinyl content from 35 to 80 mol percent based on the number of repeat monomer units in the conjugated diene block, wherein when both (i) and (ii) are present in the block copolymer composition, the ratio of (i) to (ii) is greater than 1:1.