Modification of bitumen by thermoplastic polymers plastics

Bitumen modification with thermoplastic polymers, also known as thermo-softening plastics or plastics, is not as common as bitumen modification with elastomers for paving works. 

Thermoplastic polymers are polymers that, when heated above a certain temperature (glass transition temperature, Tg), become soft and pliable or mouldable. However, when temperature drops, they harden and preserve their shape. The ability of the macromolecular chains to move and recover their original position at ambient temperatures is limited. The intermolecular bonds of the polymeric chain can absorb only vibrations and torsions and are less resistant to environmental stress cracking. As a consequence, thermoplastic polymers do not present elastic properties at ambient temperatures. 

The main thermoplastics used for the modification of the bitumen are the following ethylene vinyl acetate (EVA), polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), polystyrene (PS), ethylene methyl acrylate (EMA) and ethylene butyl acrylate (EBA). 

The effectiveness of thermoplastic polymers and, hence, the resulting characteristic properties of the modified bitumen depend on the type and quantity used, the chemical composition of the bitumen and, to a lesser extent, the manner and duration of mixing. The crystalline structure of the ethylene, responsible for the provision of hardness, constitutes the determinant factor of the copolymer behaviour (Gilby 1985 Woolley 1986). 

The addition of a thermoplastic polymer to bitumen increases bitumen viscosity and the bitumen becomes harder at ambient temperatures. Bitumen elasticity is not affected significantly. The main drawbacks of the thermoplastic bitumen are that the bitumen s elasticity is not improved substantially and that separation of the thermoplastic polymer may occur during heating. The latter is restored to a great extent by further mixing or recirculation of the modified bitumen before using it. 

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