What is the phase reversal mixture and how are regenerated polymers used in bituminous mixtures

At the beginning of the 60s of the last century, the bituminous waterproofing layer consisted mainly of bituminous paper and bitumen, in addition the bitumed glass felt had recently been introduced on the market, whose characteristics of dimensional stability and resistance were particularly appreciated.

In the same period, as we saw in the article on Giulio Natta, a whole series of stereospecific catalysts that were used to create new chemical mixtures from which new polymers, such as polypropylene, would soon be polymerized.

These new discoveries led to the manipulation of the chains of molecules creating an order of the same from the chemical point of view.

The fact of being able to position all the molecules on the same side, gives the polymer chain some advantages, including a greater mechanical resistance and a greater resistance to heat, thus defining the obtained isotactic polymer.

Its opposite, from the point of view of the position of the molecules, therefore in a disordered way, is defined as atactic.

If we want to get into some technical values ​​that characterize isotactic polypropylene we can underline that the mechanical resistance reaches up to 400 kg / sqm. and heat resistance up to 150 ° C.

The atactic polypropylene has instead the characteristic of being more malleable and elastic with elongations up to 600%.

Among the regenerated polymers and additives that are used in bitumen mixtures, there is not only polypropylene but also:

– LDPE

– HDPE

– EVA

– TPO

– Mineral Charges (talc or calcium carbonate)

– the Thermoplastic rubbers

– oils

– the waxes.

By mixing the regenerated polymers, in different percentages with the bitumen, according to the technical characteristic of the membrane to be obtained, we will have the modification of several general parameters:

– Viscosity (increases)

– The softening temperature (increases from 60 ° to 150 °)

– Penetration (decreases, then increases walkability)

– The temperature of fracture by cold bending (decreases from + 10 ° to – 20 ° eg)

– Weather stability (duration)

But what exactly happens when mixing between the bitumen and the regenerated polymers?

In technical jargon the reaction, between bitumen and polymers, during mixing is called Phase Inversion.

When a mixture is prepared, the amount of polymers is clearly lower than the amount of bitumen but, through the hot mixing of the ingredients, at a temperature higher than the melting temperature of the polymer, the so-called phase inversion takes place, where the ingredient minority, in this case the polymer, constitutes the carrier phase of the mixture, while the quantitatively majority ingredient, the dispersed phase.

To make the phase inversion take place it is important to use distilled bitumen because it is rich in oils of the maltene fraction compatible with polymers.

During the creation of the recipes, the producers of bitumen-polymer membranes use the regenerated polymers in the form of:

– Granules

– Flakes

– Densified (only if easily dispersible)

As far as granule filtration is normally required it is less than 800-1000 microns, while for the ground and the densified it is required a good degree of cleaning (washing) and a low moisture content in order not to have dangerous reactions during the mixing with the hot bitumen.

Automatic translation. We apologize for any inaccuracies. Original article in Italian.