What is Polymer Modified Bitumen?

Since many years ago engineering has met this challenge to help highway asphalt improvement including increasing Asphalt life and performance. For this aim a wide range of custom asphalts have been made and used from modified polymer bitumen.

Polymer modified bitumen is a type of bitumen that improve their chemical and physical properties by adding polymer materials to the bitumen structure. This bitumen is used to increase the resistance, quality and lifespan of asphalt layers in road construction, insulation and sealing.

 

What Polymers can be added normally to Bitumen?

Polymers that are added as additives to polymer-modified bitumen can improve bitumen properties such as adhesion, softening point, brittleness of bitumen at low temperature, etc.

Likewise, some of the polymers that are added to bitumen as modifiers are: polyethylene, polypropylene, polystyrene and polyvinyl chloride, which are thermoplastic polymers.

 

Functions of each polymer added to Bitumen:

 Adding polyethylene to bitumen will increase the softening point and decrease the permeability of bitumen.

For instance, Polypropylene will give the same properties to bitumen, with the difference that because polypropylene has a higher melting point than polyethylene, the desired properties can be obtained by using smaller amounts of polypropylene.

Moreover, Polymers that are added to polymer as additives to modified bitumen can improve properties such as softening point, adhesion, brittleness of bitumen at low temperature.

 

What are polymer materials used as bitumen modifiers:

SBS: It is a type of block polymer that consists of a combination of polystyrene and polybutadiene. This polymer makes bitumen have high elastic and viscoelastic properties and is resistant to heavy traffic, temperature changes and cracking.

EVA: is a type of thermoplastic polymer that is composed of ethylene and vinyl acetate. This polymer makes the bitumen have high adhesion and permeability properties and is resistant to moisture, corrosion and freezing.

PE: It is a type of thermoplastic polymer that is formed from the polymerization of ethylene. This polymer makes the bitumen have high hardness and resistance properties and is resistant to fatigue and scratches.

Regarding the use of this polymer in bitumen, it should be noted that the bitumen properties improve only if SBS is in a dispersed phase inside the bitumen. This increase in stability is possible in two ways.

In this method, by adding another substance to the mixture, it increases the possibility of mixing by reducing the difference in density.

Also, it is performed at the microscopic level, by reducing the size of the polymer particles to the nanometer level, it solves the problem of this incompatibility.

Meanwhile, by reducing the weight of particles against the bitumen-polymer intermolecular forces, the accumulation of polymer molecules and the formation of a polymer domain are prevented.

Another point that should be noted despite the high quality of the results in the use of these polymers is the relatively high price of these polymers compared to other types.

Theoretically, the increase in the softening point is due to the formation of three-dimensional networks in the bitumen environment after the formation of long polymer chains, which increases the elasticity and fluidity of the bitumen at high temperatures.

Obviously, this leads to better performance of bitumen at high temperature. Also, with the increase of polymer, the permeability index of bitumen increases, which indicates a decrease in thermal sensitivity of bitumen and its favorable performance in areas with large thermal differences.

 

Types of modifiers in polymer bitumen production:

• – Polymer bitumen production process with homogenous pump
• – Thermoplastics (such as polyolefins and ethylene copolymers (EBA, EMA and EVA))
• – Elastomers (such as SB and SBR styrene copolymers)
• – thermoplastic elastomers (styrene copolymers SEBS, SIS and SBS)
• – Recycled tires
• – Modification of bitumen with polyolefins:

Bitumen is brittle at low temperatures, therefore, to correct this problem and produce polymer bitumen, polymers that have a lower Tg are used in the bitumen structure. Because most polyolefins have a low Tg, the mixture of bitumen and polyolefins withstands higher strains at low temperatures.

 

Bitumen modification and polymer bitumen production with thermoplastic elastomers:

 In the production of polymer bitumen, there is another group of polymers called thermoplastic elastomers that improve the properties of polymer bitumen at low temperatures. In fact, this group of polymers does not have the problems of mixing elastomers and thermoplastics with bitumen.

Although, Thermoplastics are well-behaved during processing and mixing with bitumen, but they do not have good elastic properties in operation. On the other hand, elastomers are not ideal in operating conditions, but during mixing and shaping, they have high viscosity and make the work difficult. These problems have been solved in thermoplastic elastomers.

The most important members of the thermoplastic family of elastomers are styrene-butadiene (SB), styrene-butadiene-styrene (SBS), styrene-isoprene-styrene (SIS), etc. Styrene chains behave thermoplastic and butadiene chains cause viscoelastic properties in the product.

 

 Bitumen modification with styrene butadiene styrene (SBS)

Currently, the most common and effective polymer used to produce polymer bitumen is SBS. When SBS is mixed with bitumen, its elastomeric phase absorbs the light components of the base bitumen (such as aromatic oils) and its volume increases up to 9 times the initial volume and creates a three-dimensional network in the polymer bitumen that prevents cracking. Also, it Prevents longitudinal and transverse.

SBS family polymers affect all bitumen properties. At high temperatures, it is able to significantly reduce rutting even when the percentage of SBS in the asphalt is about 3% and the three-dimensional elastomeric network has not yet been formed. At low temperatures, it improves the flexibility of bitumen and prevents the creation and growth of cracks.

Unfortunately, when stored at high temperatures, SBS tends to separate from the bitumen, the separation of the polymer phase from the bitumen in storage tanks causes problems in pumping. Therefore, it is necessary to make SBS compatible with bitumen as much as possible. Since the higher the percentage of styrene in copolymer blocks, the greater the incompatibility of SBS with bitumen, therefore, in bitumen modification applications, grades are usually used in which styrene constitutes between 20 and 30% of the total weight of the polymer.

 

bitumen modification with ethylene vinyl acetate (EVA) copolymer

Adding EVA to bitumen improves many properties of bitumen and the production of polymer bitumen. This copolymer does not have a solubility problem in bitumen and its properties depend on the molecular mass and percentage of vinyl acetate.

In order to investigate the effect of this polymer on bitumen, tests of softening point, penetration and adhesion have been conducted for polymer bitumen modified with ethylene vinyl acetate, and the results show that the presence of 4% EVA in bitumen improves some important properties of bitumen such as stability, stiffness, adhesion and reduced penetration. EVA increases the elastic properties and reversibility of polymer bitumen, reduces the speed of crack growth, and also improves privacy.

 

Bitumen modification with recycled rubber crumb:

The use of scrap or powder of recycled tires in bitumen modification and polymer bitumen production dates back to the 1960s. In addition to its environmental benefits, it increases the viscosity of bitumen by 100 to 2000% and improves the properties of polymer bitumen in both hot and cold temperature ranges.

Some of the advantages of adding waste tires to bitumen are as follows:

• – Polymer bitumen production machine
• – Reducing environmental pollutants
• – Increased resistance to fatigue
• – Greater viscosity and adhesion and resistance to creasing at high temperatures
• – Reducing brittleness at low temperatures
• – Reduce the noise of the wheels
• – Improving the elasticity of the final coating

 

Methods of Polymer Modified Bitumen Production:

Direct mixing method:

In this method, polymer material is added to bitumen in the form of powder or granules and mixed with heat and stirring. This method is simple and cheap, but it may cause polymer lumps in the bitumen.

Indirect mixing method:

 In this method, the polymer material is first dissolved with a solvent or another binder such as bitumen modified with motor oil (MBO) or bitumen modified with vegetable oil (VBO) and then mixed with bitumen. This method allows the polymer to disperse well in the bitumen, but requires the use of additional solvents and binders.

Mechanical mixing method:

 In this method, polymer material is added to bitumen in the form of film or fiber and mixed using special devices such as extruder or calender. This method allows the polymer to be distributed uniformly and regularly in the bitumen, but it requires complex and expensive equipment.

Polymer bitumen production process

Since this mixture is actually a two-phase mixture, the smaller the size of the dispersed polyolefin particles in the polymer bitumen, the greater the resistance to crack growth, and as the coarser the dispersed polyolefin particles, this resistance decreases. In other words, the larger the distance between polymer particles and the larger the particle size, the more brittle behavior is observed.

Therefore, one of the essential points in modifying bitumen with polyolefins is to use suitable mixing equipment that reduces the size of polymer particles inside the bitumen matrix as much as possible.

 

Mixing process in Modified Bitumen Production:

 One of the determining factors in the effectiveness of polymers in bitumen modification is the mixing process. If the polymer is not properly mixed with the bitumen, the morphology of the final polymer bitumen mixture will not be uniform and the phase difference will be observed. This means that the network structure that causes the storage of deformation energy and recovery of the initial state, especially at high temperatures, will not be obtained.

 

Homogenizers:

Shear homogenizers play a vital role in the production of polymer bitumen, because they easily disperse solid and liquid particles in the bitumen phase. High shear homogenizers used in the production of polymer bitumen have a rotor and a stator.

 

Homogeneous pump performance in Production of Modified Bitumen:

 After entering the homogenizer pump, the material passes through the space between the rotor and stator rows, which is a very small distance. The number of stator rotor rows can be changed from 3 to 15 rows. The higher the number of rows of rotor and stator, the more the fluid is subjected to shear stress once it passes through the pump, and in this way, the crushing and homogenization of materials is done with greater speed and efficiency.

The distance between each row of rotor and stator is called a stage. For example, when the fluid passes through a pump with 8 rows of stator rotor, it undergoes 7 stages of shear stress and passes through 7 stages and then exits the pump. Such a homogenizer pump is called a seven-stage homogenizer pump.

 

Jacketed homogeneous pump for the production of polymer bitumen:

 Homogeneous pumps for the production of polymer bitumen have a hydrostatic jacket to be able to heat or cool. In most cases, this jacket is used for heating. And before starting the production process, the temperature of the jacket reaches the melting temperature of the polymer bitumen, and then the pump is turned on. Because the manufacturer is worried that by starting a pump that has not been properly cleaned, there is a possibility that the bitumen mixture will remain inside the stator rotor, and when the pump is turned on, it will put pressure on the engine.

Although the probability of this phenomenon is very low, there is always the possibility of human error. Because the operator of the machine may not have washed the pump and the circulation path properly in the previous shift, and a small amount of material remaining on the path or inside the pump, and its freezing at ambient temperature, exerts a lot of pressure on the pump during startup and causes damage. to be pumped

As the material undergoes high shear stress when passing through the homogenizer, it heats up inside the pump. This heat causes the temperature of the fluid to rise above the required level while passing through the pump and the possibility of breaking the polymer chains in the mixture. Therefore, it is suggested that after starting the device, the heating of the jacket should be turned into cooling by cold water.

Important points in the production process of polymer bitumen with a homogenous pump:

As a matter of fact, the production of polymer bitumen is important and decisive in the structure of asphalt roads, and it is also widely used in roof insulation. Asphalt mixture consists of aggregate, polymer bitumen and empty spaces, which includes about 94-96% aggregate and 4-6% bitumen.

Meanwhile, polymers are one of the most important bitumen modifiers. Depending on their type, polymers will have positive effects on the performance properties of polymer bitumen production.

Hence, Polymer bitumen has high capabilities at high and low temperatures, including resistance to rutting and thermal cracks.

Not only improving the properties of bitumen and producing polymer bitumen will increase the quality level but also it increases its useful life up to 25% (between 2 and 10 years). As a result, the costs of maintaining and covering asphalt mixtures will be significantly reduced.

 

Polymer Modified Bitumen (PMB) Applications:

Because of the high volume of traffic on the roads, it is difficult to repair the damaged areas, especially in bridges, for this reason, the modified polymer bitumen in the preparation of asphalt contributes to the resistance of the asphalt and the need for repairs.

Relatively, the result of the penetration degree, softening point and viscosity tests showed that the addition of SBS polymer to bitumen improves the rheological properties of bitumen.

Chiefly, it is suitable to use asphalt containing bitumen modified with SBS polymer in areas with hot summers and cold winters or large temperature differences between day and night.

In hot weather, the reduction of the penetration of this bitumen and its non-viscosity along with the high strength of the mixture make it possible to withstand the load without creating grooves.

The excellent tensile strength along with the coefficient of growth of the asphalt mixture with the elastic properties of bitumen creates good flexibility so that this mixture can be used in cold weather when the surfaces are prone to cracking due to shrinkage.

Consequently, the use of this bitumen in asphalt, despite the higher initial costs in its production, has an economic potential due to the increase in pavement life with increased durability and indirect savings in several times the cost of improvement and repair.

Written and gathered by Pars Bitumen