Characteristics of polymer modified bitumens
In the case of polymer-modified bitumen, it can be said that with the increasing traffic of cars, the usual street coverings no longer have the necessary efficiency for the safe and comfortable transportation of vehicles. Because this volume of traffic greatly reduces their service time, and as a result, the cost of road maintenance and staining also increases.
For this reason, modifying asphalt and increasing its durability is considered as one of the most important issues in the field of road construction. Among the proposed options in this field are polymer-modified bitumens (PMBs), which are produced in order to increase the performance and durability of asphalt materials and also to increase the adhesion of these materials to each other.
PMBs are produced by mixing bitumen and polymer (usually 3% to 7% by weight) by dispersing polymer in molten bitumen. Approximately 75% of PMB is elastomeric, 15% is plastomeric and the rest is rubber or a combination of the three.
Needless to say, increasing the useful life and better quality of PMB leads to the improvement of the economic requirements of maintenance and road safety, which can overcome the higher cost of the initial investment. But the preparation of the desired PMB is not easy due to the difficult requirements that the modified asphalt must have, including suitable mechanical properties, favorable stability during storage, viscosity compatible with the devices and common road construction processes at high temperatures, as well as being economically viable. is not.
Despite all these requirements, the availability of various types of polymers increases the possibility of achieving this goal (making polymer-modified asphalts). The main reason for focusing on polymers for asphalt modification is their ability to produce physical networks that have both crystalline and flexible parts.
In general, the polymers used to modify asphalt are usually divided into three categories:
⦁ thermoplastic elastomers,
⦁ Plastomers and
Active polymers
History
Modified bitumens with synthetic and natural polymers were introduced in 1843. Until the 70s, Europe was a pioneer in using PMB. However, due to the high cost of PMB, its use in the United States was very limited. The bitumen compound based on bitumen and polyisobutylene was created in 1940.
After the crisis of 1973 and 1979, the use of modified polymer bitumen for the road construction industry increased gradually over a period of 40 years. In 1970, researches were conducted to produce bitumen additive polymers including plastomers and thermoplastic elastomers. The results of those researches were able to improve some of the properties of bitumen, such as reducing the sensitivity temperature or increasing the bond's resistance against permanent deformation.
In the mid-1980s, the United States began using new advanced polymers and European technologies to produce PMB. Bowering reviewed the requirements of polymer bitumens in 1984. He claimed that the relatively high cost of PMB may be justified compared to the reduction of costs caused by the reduction of the layer thickness and the increase in the life of PMB. In 1989, Reese et al. in their study observed the high resistance of PMB to aging and failure after a two-year period in California.
In 1997, assessments conducted by state departments of transportation indicated that 47 of the 50 states in the United States were willing to begin using modified couplings in the future, and 35 states called for large-scale use. The United States, China, France, and Italy are the pioneers of research in the field of PMB and are ahead of countries such as Japan, Germany, Russia, Great Britain, and Canada.
Features of using modified polymer bitumens
Many systematic researches have been carried out on features such as rheology, temperature sensitivity, morphology, temperature behavior, stability and aging in different PMBs. The result of these researches is to identify the advantages and disadvantages of using PMBs.
In general, polymer modified bitumens improve some of the following properties:
⦁ improving the elastic reaction,
⦁ High resistance to failure at low temperature and
⦁ High resistance to the falling line of the wheels of heavy vehicles at high temperature in bitumen modified with SBS.
⦁ thermal instability of some bitumen modified with polymer,
⦁ phase separation of some PMBs and
⦁ High adhesion to storage tanks and transport trucks and the difficulty of cleaning their surfaces after use.
Many efforts to overcome the disadvantages of PMBs started in 1990. In 1996, Giavarini et al claimed that polypropylene-modified bitumen could be stabilized by adding polyphosphoric acid (PPA). They believed that PPA can help to improve the storage stability of PMB by changing the bitumen structure from sol to gel. But the most important point in this regard is to examine the theory to understand the behavior of bitumen during its separation.
In most cases, the failure of bitumen in road construction can be checked through the adhesive properties of bitumen. Investigating these characteristics theoretically can be done using boundary, mechanical, chemical, electrostatic and thermodynamic theories. Many researches have been dedicated to investigating the adhesion of bitumen components in the vicinity of water. But before using these theories, a comprehensive understanding of bitumen chemistry is required, which will be discussed further.
Chemistry of polymer bitumens
It has been shown in some researches related to the structure, mineral bitumens have a heterogeneous colloidal structure and PMBs should have a multiphase emulsion (polymer-asphaltene-malten). In some other studies, it has been claimed that bitumens are homogeneous. It should be noted that the polarity of polymers also has a specific effect on their compatibility with bitumen and, finally, on the storage stability of the resulting PMBs.
Today, bitumen molecules are divided into two main categories: asphaltenes and maltenes. Maltans themselves are divided into three groups of saturated substances, aromatics and resins. For this reason, SARA components are defined as saturates (S), aromatics (A), resins (R) and asphaltenes (A), whose molecular weight, polarity, and non-aromatic content increase as S<A<R<A.
These four constituent components of bitumen interact differently with the polymer, which usually leads to the swelling of most of the modified bitumen molecules and the creation of polymer phases and bitumen residue. As a result, a two-phase morphology with polymer-rich phase (PRP) and bitumen-rich phase (ARP) is formed.
The structure of PMB can be examined using micrography. Increasing the amount of polymer (from 4% to 8% by weight) has caused a significant change in the shape of new network particles with at least three continuous matrix phases of remaining bitumen components, polymer particles surrounded by aromatic components and solid asphalt particles. It is assumed that the polymer network formed in such a multiphase system is based on microcrystals in which several polyethylene chains are connected through the bitumen matrix and act as a continuous bitumen phase reinforcement.
Of course, PMB can have other structures as well. For example, in the investigation of bitumen mixture and styrene-butadiene random batch copolymer (SBR) with the same percentage of polymer, a different structure is observed. SBR can be seen as a fibrous structure and due to its amorphous structure and rubbery behavior, it creates a three-dimensional network within the continuous bitumen phase.
Increasing the percentage of polymer also has no noticeable effect on the distribution state. Usually, asphaltenes do not have a tendency to swell with polymer. Depending on the swelling ability and operating conditions (time, temperature, shear stress, etc.), several different morphologies are created in polymer bitumens. Only when the amount of polymer swelling by bitumen molecules is high enough, PRP can be a continuous phase, which is related to phase inversion.
In this case, the polymer can have a strong effect on the bonding properties, or in other words, it can improve the performance properties of bitumen. The most favorable case is the occurrence of full inflation, in which case ARP can disappear. Of course, provided that this swelling is not accompanied by the destruction of the polymer network.
From a functional point of view, the minimum required inflation is the amount that guarantees phase inversion. At this minimal swelling, the polymer maintains its original structure and most of the general properties remain similar to those of the intact (non-swollen) polymer.
On the other hand, relatively low swelling is achieved only when most of the compatible components of the bitumen migrate into the PRP. This internal migration shows a noteworthy difference between PRP and ARP components. In such a case, PRP and ARP have a strong tendency to separate macroscopically, which is caused by their density difference. If this separation occurs in storage tanks or transport trucks, the homogeneity within the material is completely lost.
In this case, PRP becomes so viscous that conventional pumping equipment will not be able to suction it. Therefore, selectivity in swelling and migration of bitumen molecules in PRP should be kept low enough.
At low swelling values, PRP is dispersed in ARP and the polymer does not have a noticeable effect on bonding behavior (the properties are similar to pure bitumen). At the point of phase inversion (PI), a sudden change in the characteristics of the joint is observed. But in this case, the system does not have the necessary stability to maintain. A further increase in the degree of swelling causes a slight decrease in performance, but this amount is still much higher than that of the base bitumen.
As the degree of swelling increases, the mixture enters the complete solubility (CS) region. In this region, the system becomes a homogeneous mixture and again, the effect of the polymer becomes negligible.
It should be emphasized that due to reasons such as bitumen formulation, type and quantity of polymer and mixing conditions, it is very difficult to achieve complete dissolution in most cases.
In addition, the mixing time should not be so much that bitumen production is not economical. On the other hand, high shear-high temperature mixing process can cause degradation or aging of polymer and even bitumen. In other words, the time and method of mixing has an effect on molecular weight, polymer chemistry and bitumen.
In order to achieve the optimal product, in addition to thermodynamic and kinetic issues, production conditions should also be taken into consideration. As a general principle, compounds with similar polarities are miscible.
This principle should be considered more about compounds with high molecular weight. To convert this qualitative principle into a quantitative expression, Hildebrand and Scott first used the square root of the adhesion energy density. This parameter was later divided by Hansen into three terms: atomic non-polar interactions, molecular dipole and molecular hydrogen bonding.
As a result, its elasticity recovery capacity and resistance to permanent deformation increases. On the other hand, if the bitumen molecules are separated from the glass domain, the lattice vibrations are lost and the bitumen modification process enters the CS region in Figure 4.
Because the temperature during modification is just above the Tg of the hard part and the network is likely to be destroyed, such a morphology during the modification process is not derived from simple swelling of the soft phase with an intact and unswollen hard phase, but rather gives the possibility The hard sphere may swell or melt during mixing.
The mixture is regenerated when it cools below the Tg temperature. Since the hard domain consists of an independent phase, the most appropriate example is a three-phase system in which PRP is divided into two subphases consisting of soft swollen parts and partially swollen hard parts.
Poly(styrene-b butadiene-b styrene) block copolymer (SBS) is very preferable for bitumen modification. The glass transition temperature of the soft part (Tg) is negative and the hard part is positive. In addition, poly(styrene-b isoprene-b styrene) (SIS) and poly(styrene-b ethylene-butadiene-b styrene) (SEBS) obtained from the hydrogenation of SBS are also used for this purpose.
Smith et al investigated the idea of producing modified bitumens using active polymers. They used two main components and fillers to produce this type of polymer asphalt.
Its first component consisted of 25% to 75% by weight of bitumen, coal tar, or a combination thereof, and the second component was polyurethane produced from the reaction of polyisocyanate compounds and polyol with an equal ratio or a mixture of polyurethane and rubber (SEBS), SBS or (SIS). The isocyanate group at the end of polyurethane reacts with the hydroxyl group at the end of bitumen and coal tar.
The resulting formulation can act as a membrane for keeping minerals. 1% to 66% by weight of fillers also included calcium carbonate, talc, ammonium polyphosphate, etc. This formulation was produced by melting the first component, adding other compounds to it and mixing.
Common plastomers used in modified bitumens are polyolefins such as polyethylene and polypropylene. Since polyolefins cannot be directly added to bitumen due to their non-polar nature, compatibilizers are added to them.
Common compatibilizers have epoxy functional groups such as glycidyl methacrylate (GMA) or glycidyl acrylate or maleic anhydride. Other oxygenated functional groups, such as acrylic esters, can also be added to bitumen.
These modifiers are known as active ethylene terpolymers. The proposed interaction mechanism for these modifiers includes physical and chemical mechanisms. The physical interactions are similar to the PB part in SBS, which swells due to penetration.
Chemical interactions are mainly carried out by GMA or MAH and lead to the networking of the bitumen structure. The importance of this category of polymers lies in the limitation of their usage amount (1.2% to 5.5%). If the amount of polymer is more, it also creates a cross link between the polymer chains, which results in the creation of an insoluble and impermeable gel.
Improving the performance of polymer bitumens
Since 2000, many efforts have been made to overcome the disadvantages of polymer bitumens, among which the following methods can be mentioned:
1- Vulcanization using sulfur:
Due to the low compatibility of bitumen and styrene-butadiene-styrene (SBS) copolymer and in order to increase the stability of SBS-modified bitumen, Wen et al. used sulfur vulcanization method at 180 °C. In this process, sulfur reacts with naphthenes and aromatic groups and Ar-(S)X-Ar bond is created. As a result, the physical and mechanical properties of bitumen-sulfur mixture are improved.
2- Antioxidant increase:
Oxidation changes the rheological properties and consequently the physico-chemical properties of bitumen. In this regard, Dessouky et al. used 2% by weight of sterically hindered phenols and 3% by weight of SBR (styrene-butadiene rubber) as antioxidants in order to increase the resistance of PMB against oxidation caused by contact with air oxygen at high temperatures. Their observations showed that, in addition to increasing resistance to oxidation, the strength of PMB also increased significantly.
3- Use of hydrophobic mineral clay:
Zhang et al produced bitumens modified with montmorillonite and SBS by melt blending to increase the UV resistance of PMB. Also, after aging, the viscosity and softening point increase with the increase of soil.
4- Functionalization (including the use of active polymers):
Shivokhin et al showed by adding 4% by weight of 4,4-diphenylmethane diisocyanate (MDI) polyethylene glycol (PEG) to bituminous mastic modified with 3% by weight of SBS, the viscosity at different cutting speeds and at °C temperature. C60 remains almost constant.
In order to reduce costs, suggestions such as the use of waste tires and plastics and by-products of plant-based polymers have been presented to be considered in future works. Crumb rubber (CR), which is obtained from scrap tires of passenger cars and trucks, contains 50% rubber, 20% carbon mass, 15% metal and 15% other materials.
The interactions of CR with bitumen are mostly physical. But chemical interactions are not impossible either. Chemical reactions between rubber and bitumen can be expected when the temperature is in the range of 170-200 °C.
Conclusion
Modified polymer bitumens, despite having noteworthy advantages, in most cases, have not yet achieved sufficient acceptance. The chemistry, structure, and properties of these bitumens are highly dependent on their components, compatibility with the used polymer, and production conditions.
In order to achieve the desired bitumen, the desired swelling of the polymer, creating and maintaining the modification properties and preventing the biphasing of the product (stability) is necessary. The presence of different components with different polarizations and solubility, the high cost and the possibility of phase separation of these bitumens during temperature changes in the operating conditions and under the traffic load of cars are the major challenges of increasing the acceptability of this type of bitumen.
In general, there are several general methods to solve these problems, which require more detailed investigation and more operational tests, considering the priority of reducing the total cost or improving the production bitumen performance.
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