Polymer modified bitumen

Basic Properties of BITUMEN:

Bitumen is so useful in the road making and road maintenance industries because of its basic thermoplastic nature, i.e. it is stiff/solid when cold and liquid when hot, (well with penetration grade bitumens anyway).(The modifying polymers used in bitumen are also thermoplastic in nature.)The basic properties of bitumens can be modified by the addition of flux oils or volatile oils to produce bitumens of various grades.These grades are specified by their viscosity, (penetration), and their softening point, this information, along with other physical characteristics is specified in,BS EN 12591 : 2000 : Bitumen and bituminous binders - Specification for paving grade bitumens This standard superseded BS 3690:Part 1, which is still often referred to.

The above ways of altering the characteristics of bitumen are really ways of decreasing the stiffness of the binder and increasing the workability of bituminous mixtures at lower temperatures, e.g. hand-lay work.Of course penetration grade bitumens modified with flux oils or volatile oils will have a lower performance in use. It is a good policy to always try and use the highest/stiffest grade of bitumen in a particular mix consistent with being able to lay and compact it efficiently. Some bitumen may require modifiers, such as polymers, to meet low and high temperature requirements. Although modifiers may affect many properties, the majority of modifiers attempt to decrease the temperature dependency and oxidation hardening of bitumen and asphalt mixtures.

Polymers used for bitumen modification

Polymers used for bitumen modification are long chained hydrocarbon molecules that enhance the properties of neat bitumen. Depending on the basic polymer units (monomers) used, a wide range of properties can be achieved. It is possible to categorize polymers in a number of ways, but for engineering purposes they are conveniently described as having glassy (stiffness) or rubbery (elastomeric) properties. Often this is termed plastomeric or elastomeric. Plastomers will deform but will not return to their original dimensions when the load is released. Elastomers will deform and return to their original dimensions when the load is released, however, this is very dependent on conditions such as temperature, rate of loading and strain level. As the demands of a modern road system have, in some areas, exceeded the capacity of conventional bituminous materials, polymer additives are a means by which pavement performance may be enhanced.

The most commonly used polymers to manufacture polymer modified bitumen are: styrene-butadiene-styrene (SBS) block copolymers (elastomeric) synthetic styrene-butadiene-rubber (SBR) latex (elastomeric) ethylene-vinyl-acetate (EVA) block copolymers (plastomeric) natural rubber latex (elastomeric) atactic polypropylene (APP) (plastomeric) high molecular weight waxes from the Fischer-Tropsch process (mainly as warm asphalt additives)POLYMERS

Thermoplastic Rubbers, (TR's)

This may be regarded as a group name / description for a number of polymers/copolymers used in the modification of bitumen.A copolymer is a polymer that has more than one type of molecule incorporated in the polymer.These polymers are made up of many thousands of individual monomers/molecules built up into chains by the various polymerisation processes developed by the large chemical industries.

Styrene Butadiene Styrene, (SBS)

This is a thermoplastic rubber.SBS is a copolymer that you will come across in bitumen modification, it was originally developed for use in the production of tyres and the soles of shoes, but is suitable for the modification of bitumen.

Ethylene Vinyl Acetate, (EVA)

This is not regarded as part of the thermoplastic rubber group but is still thermoplastic in its nature.One of the uses for this type of polymer are the "hot melt" glues, the sticks of which you may be familiar with in "D.I.Y" hot melt adhesive guns.

The most common grade of EVA for bitumen modification, for road pavement materials, is the classification "150/19". This classification means it has a melt flow index of 150 and a vinyl acetate content of 19%, how much you include in the bitumen to be modified for optimum benefit can be debatable, but 5% by weight is a commonly quoted figure.

EVA modified mixes have been around for some time now, I can remember them being used 25 years ago, and at that time EVA incorporation was claimed to make to make HRA wearing course more workable in cold weather. In fact EVA was being added to 70 pen binder to produce a wearing course asphalt which indeed was much more workable in cold conditions.

The theory was good, because at higher mix temperatures the EVA does not increase the stiffness of the mix but at lower temperatures the EVA polymer recombines to increase the stiffness of the asphalt above that expected from a 70pen. bitumen.

But, it did cause all kinds of problems with rolling, one of the complications being chippings "lost" in the mat due to the initial reduced stiffness of the material, the other being differential cooling so that in some areas of the laid bituminous mat the EVA had recombined and in other areas it had not.The differing rates of cooling produced different levels of stiffness in the mat and this in turn has been known to cause "tearing" in the mat whilst rolling Out of this list, the most common polymer for bitumen modification is styrene-butadiene-styrene block copolymer.

Polymer modified bitumen manufacturing

There are many types of manufacturing configurations to make polymer-modified bitumens (PmB). Manufacturing may be done at high or low shear, on site, or in a factory. The main stages of manufacturing require the following procedures:

Metering of polymer, bitumen, and additives Wetting of the polymer by the bitumen/additive mix Dispersion of the polymer Allowance for any interaction (reaction) of the polymer with the bitumen Storage and transportation

Most of these are mechanical issues and are achieved by relatively simple techniques. The most important steps in the manufacturing process are dispersion and reaction. This is what determines the structure (i.e., morphology) of the final binder and hence its properties. These steps also determine the level of polymer required to achieve the desired results. Compatible systems usually have superior rheological, aging, and stability properties to those of incompatible systems at the same polymer level.

Performance of polymer modified bitumen

Polymer modified bitumen have the ability to offer improved performance over conventional, non modified bituminous binders, but are economical considerations should be normally taken to ensure their higher price (compared to non modified binders).

Benefits that may be derived from bitumen modification include:

improved consistency, reduced temperature susceptibility, improved stiffness and cohesion, improved flexibility, resilience and toughness, improved binder-aggregate adhesion, improved resistance to in-service ageing, improved rutting resistance.

compositional groups:

From chemistry aspect, crumb rubber modifier (CRM) is a polymer, and asphalt rubber a polymer modified bituminous binder, however in common industrial practice modified bitumen have been divided into the following two broad compositional groups:

1. Homogenous polymer modified bitumen which are defined as a blend of polymer and bitumen where two distinct phases are interwoven to such an extent that the material behaves as a single-phase material from a small, localized viewpoint. Examples of homogenous binders are EVA, SBR and SBS polymer modified bitumen.

2. Non-homogenous polymer modified bitumen are where there are two distinct, detectable phases and where there will be localized differences in properties depending at what stage a test is performed. Asphalt rubber, rubber modified bitumen falls into this category as it consists of rubber crumbs partially dissolved in a bitumen matrix.

THE WAYS THE ADDITION OF POLYMERS MAY MODIFY BITUMEN

The polymer additives do not chemically combine or change the chemical nature of the bitumen being modified, apart from being present in and throughout the bitumen.What polymers will do is change the physical nature of bitumens, and they are able to modify such physical properties as the softening point and the brittleness of the bitumen. Elastic recovery/ductility can also be improved.

This in turn will alter the properties of the aggregate / bitumen mixture in which the modified bitumen is used.These criteria are important in a mix with regard to problems such as wheel track rutting at high temperatures and fatigue cracking at low temperatures due to the brittleness of the mix.

The basic laying workability of the asphalt or macadam you are using will still be governed by the viscosity of the grade of bitumen you have specified. It is usually the stiffer 50pen, or possibly 70pen bitumen that is modified.

The way the additive/polymer usually influences the bitumen characteristics is by dissolving into certain component fractions of the bitumen itself, spreading out its long chain polymer molecules to create an inter-connecting matrix of the polymer through the bitumen. It is this matrix of the long chain molecules of the added polymer that modifies the physical properties of the bitumen.

Because of the thermoplastic nature of the polymers, some polymers will actually break up into their constituent molecular blocks at the high temperatures, during mixing and laying, and recombine into their polymer chains at lower temperatures, i.e. ambient temperatures.

What has to be ascertained in practice is the degree of modification that takes place, and whether the degree of improvement achieved in the overall qualities of the bituminous mix is worth having, and is it cost effective.