European Polymer Journal 54 (2014) 18–38

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European Polymer Journal

journal homepage: www.elsevier .com/locate /europol j

Review article

Polymer modification of bitumen: Advances and challenges

http://dx.doi.org/10.1016/j.eurpolymj.2014.02.0050014-3057/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +46 (0)8 790 8707.E-mail address: jiqing.zhu@abe.kth.se (J. Zhu).

Jiqing Zhu ⇑, Björn Birgisson, Niki KringosDivision of Highway and Railway Engineering, Department of Transport Science, KTH Royal Institute of Technology, Brinellvägen 23, 100 44 Stockholm, Sweden

a r t i c l e i n f o a b s t r a c t

Article history:Received 9 September 2013Received in revised form 22 January 2014Accepted 5 February 2014Available online 17 February 2014

Keywords:Polymer modified bitumenAdvanceChallengeFuture development

Advances and challenges in the field of bitumen polymer modification for road construc-tion during the last 40 years are reviewed in this paper. The history of bitumen polymermodification is described chronologically. Some popular plastomers and thermoplasticelastomers in bitumen modification are discussed regarding their advantages and disad-vantages, including polyethylene (PE), polypropylene (PP), ethylene–vinyl acetate (EVA),ethylene–butyl acrylate (EBA), styrene–butadiene–styrene (SBS), styrene–isoprene–sty-rene (SIS) and styrene–ethylene/butylene–styrene (SEBS). Although these polymers allimprove bitumen properties to some extent, there are still some drawbacks limiting thefuture development of bitumen polymer modification, such as high cost, low ageing resis-tance and poor storage stability of polymer modified bitumen (PMB). Researchersattempted various ways to remove these drawbacks. Some technical developments forremoving drawbacks are reviewed in this paper, including saturation, sulfur vulcanization,adding antioxidants, using hydrophobic clay minerals, functionalization and application ofreactive polymers. The future development of polymers for bitumen modification is ana-lyzed as well. Since it is currently challenging to perfectly achieve all expected PMB prop-erties at the same time, some compromised recommendations are given in this paper,among which greatly enhancing the properties with an acceptably high cost, significantlyreducing the cost with relatively poor properties and their combinations. Functionalizationis emphasized as a promising way to enhance the properties of currently used polymersand develop new-type polymer modifiers with much greater success in the future. It is alsorecommended that future research on bitumen polymer modification focuses more onfunction development towards enhancing: adhesion with aggregates, long-term perfor-mance and recyclability.

� 2014 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192. Historical perspective. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203. Popular polymers for bitumen modification. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.1. Plastomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223.2. Thermoplastic elastomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4. Technical developments for removing drawbacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4.1. Sulfur vulcanization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274.2. Antioxidants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

J. Zhu et al. / European Polymer Journal 54 (2014) 18–38 19

4.3. Hydrophobic clay minerals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294.4. Functionalization and reactive polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

5. Future developments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306. Conclusions and recommendations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

1. Introduction

Bitumen is one of the oldest known engineering materi-als [1]. It has been used for thousands of years [2] in vari-ous ways, e.g. as adhesive, sealant, preservative,waterproofing agent and pavement binder. Ancient inhab-itants directly used the natural bitumen which is usually inthe earth’s surface [2]. In the early 1900s, refined bitumenwas first produced by refining crude oil in the USA [1].Since then, the world consumption of bitumen has in-creased rapidly, most of which was used in road construc-tion. According to a joint publication of Asphalt Instituteand Eurobitume in 2011, the current world consumptionof bitumen is approximately 102 million tonnes per year,85% of which is used in various kinds of pavements [3].In fact, the chemistry composition of produced bitumenis very complex and variable; and the properties of pro-duced bitumen are closely related to the crude oil sourcesand the refinery processes. By selecting good crude oil orproper refinery processes, some good bitumen propertiescan be obtained. However, the limited oil resources forproducing good-quality bitumen and the lack of effectivecontrol actions during refinery, as well as the driving forceof earning the maximum economic benefits, made indus-tries pay more attention on bitumen modification [4].Additionally, pavement industry has developed rapidly allover the world during the last few decades, especially indeveloping countries. Following the rapid development, in-creased traffic load, higher traffic volume, and insufficientmaintenance led to many severe distresses (e.g. ruttingand cracking) of road surfaces. The harsh reality wasdemanding more on bitumen quality. In order to obtainbitumen with enhanced quality, an increasing number ofinvestigations also began to focus on bitumen modifica-tion. Among all attempted or investigated modificationmethods of bitumen, polymer modification has been oneof the most popular approaches.

Polymer modification of bitumen is the incorporation ofpolymers in bitumen by mechanical mixing or chemicalreaction [5]. During the last 40 years, more and moreresearchers began to concentrate themselves on polymermodification of bitumen and a rapidly increasing numberof research articles have been published since 1970s. Inthese, the various investigated polymers included plastom-ers (e.g. polyethylene (PE), polypropylene (PP), ethylene–vinyl acetate (EVA), ethylene–butyl acrylate (EBA)) andthermoplastic elastomers (e.g. styrene–butadiene–styrene(SBS), styrene–isoprene–styrene (SIS), and styrene–ethyl-ene/butylene–styrene (SEBS)) [6–12], although none ofthese were initially designed for bitumen modification.These polymers were reported to lead to some improvedproperties of bitumen, such as higher stiffness at high

temperatures, higher cracking resistance at low tempera-tures, better moisture resistance or longer fatigue life[13–18]. In [2], an extensive summary was given that aneffective polymer modification results in a thermodynam-ically unstable but kinetically stable system in which thepolymer is partially swollen by the light components ofbitumen. Some important factors, including the character-istics of the bitumen and the polymer themselves, thecontent of polymer and the manufacturing processes,determine the final properties of polymer modified bitu-men (PMB) [5,19]. As polymer content increases, phaseinversion may occur in some PMBs: from bitumen beingthe dominant phase to polymer becoming the dominantphase [20]. However, an ideal microstructure for PMBcontains two interlocked continuous phases, whichdetermines the optimum polymer content for bitumenmodification [21]. With these two interlocked continuousphases, PMB usually shows better overall performancewith respect to mechanical properties, storage stabilityand cost-effectiveness.

In addition to the reported advantages, researchers alsoencountered various challenges, including high cost, somePMBs’ high temperature sensitivity, low ageing resistance,poor storage stability and the limited improvement in elas-ticity. In this, the combination of bitumen oxidation andpolymer degradation was reported to cause PMB’s ageingpropensity [22], which seems especially challenging forsome unsaturated polymers, e.g. SBS. The poor storage sta-bility of some PMBs usually results from the poor compat-ibility between polymer modifiers and bitumen which iscontrolled by polymers’ and bitumen’s different propertiessuch as density, molecular weight, polarity and solubility[23]. The chemical structure and reactivity of polymers,however, are also supposed to affect their compatibilitywith bitumen, which may have a direct relationship withthe resulting PMB properties [24]. In order to conquerthese challenges, researchers have tried different catego-ries of solutions, such as saturation, sulfur vulcanization,adding antioxidants, using hydrophobic clay minerals,functionalization and application of reactive polymers(which also can be considered as new functionalizedproducts).

Along with technical aspect, economical aspect is ofcourse a huge driving force for the choice of technology.Different kinds of pavements have different demands onperformance. From the economic aspect, it is not alwaysbetter to achieve higher performance for a road. Only whenthe technology is cost-effective, can people get the maxi-mum benefits from it and can it become popular. As forPMB, the cost is quite relevant with the dosage of theadded polymer, while the polymer dosage usually hasimportant influences on the final degree of PMB

20 J. Zhu et al. / European Polymer Journal 54 (2014) 18–38

performance. So before constructing a road, the designersmust know what is the needed degree of performance forthe road and then decide to use PMB or not, and use howmuch. Currently, most of the world consumption ofbitumen is still base bitumen. As the climate and trafficconditions vary in different countries, the percentage ofPMB in all the used bitumen also varies in differentcountries. Even for a single country, the percentage variesduring different years. According to the data releasedby European Asphalt Pavement Association (EAPA), thepercentage of PMB consumption in all the yearly used bitu-men for paving is usually less than 20% in most Europeancountries during the last 3 years [25]. The detailed datafor each country can be seen in [25]. Regarding thepolymer dosage, Eurobitume claimed that a typical SBSpolymer content is around 3.5% by weight in the finalproduct, based on an internal industry review relatingPMB within Europe [26].

This paper focuses on bitumen polymer modification forroad construction, aiming to give a comprehensive over-view of the development of bitumen polymer modificationover the last 40 years, the challenges people encounteredand the solutions researchers came up with as well as theirvarying success. First, a historical perspective is given inthe following with an in-depth discussion on the mostpopular polymers and their associated technical develop-ments. After this, the potential development of bitumenpolymer modification in the future is analyzed. Finally,some conclusions are presented and some recommenda-tions are given.

2. Historical perspective

Bitumen polymer modification has a long history. Evenbefore refined bitumen was produced, people began tomodify natural bitumen and some patents were grantedfor natural rubber modification [1,27–29]. Synthetic poly-mers, however, were not widely used until after WorldWar II ended. One well-known early example is neoprene(polychloroprene) latex, which began to be increasinglyused for bitumen modification in North America from the1950s [29].

Plastomers have a longer history of artificial synthesisthan thermoplastic elastomers. Most of the currently pop-ular plastomers began to be produced commercially before1960 [30]. Regarding thermoplastic elastomers, the firstcommercially acceptable SBS product was developed inthe USA in 1965 and the first hydrogenated (or saturated)product, SEBS, was announced in 1972 [31]. In the earlyyears, these commercial polymers were mainly used inpackaging, rubber, footwear or adhesive industries.

Bitumen polymer modification was firstly used in theroofing industry, and then the paving industry. In 1965,atactic polypropylene (APP), which is a by-product of iso-tactic polypropylene (IPP) manufacturing, was firstly usedto modify bitumen for roofing in Italy and the first com-mercial product was marketed in 1967 [32]. SBS, however,was not widely used until the early 1970s in Europe. As forthe USA, it was in 1978 that Americans began to widely usemodified bitumen in roofing. Around 1980, the firstAmerican PMB manufacturer started [32].

Bitumen polymer modification for road construction isa field extensively covered by intellectual property. A pat-ent, relating a bituminous composition with base bitumenand polyisobutylene, was granted as early as 1940 [33].After that, especially after SBS was introduced to bitumenmodification, a large number of patents were applied allover the world. Due to the oil crises of 1973 and 1979, at-tempts of bitumen polymer modification for road con-struction began to increase about 40 years ago [34,35].During the 1970s, researchers proved that the addition ofpolymers, including plastomers and thermoplastic elasto-mers, could improve some properties of paving bitumen,such as reducing temperature sensitivity or increasingthe resistance to permanent deformation [35–39]. In1978, Chaffin et al. [39] reported the potential storage sta-bility problems of bitumen modified with elastomers, butthey also wrote that their field test sections constructedin Texas in 1976 were performing well.

During the 1980s, the demand of thin layer for pave-ment drove more systematic investigations [34,40–47] tofocus on bitumen polymer modification. For example,in 1980, the research carried out by Piazza et al. [40]revealed the features of bitumen respectively modifiedby plastomers and thermoplastic elastomers. In 1982,Kraus [41] studied the morphology of modified bitumenby elastomers and reported the swelling of polymers inbitumen. In 1983, a binder for pavement wearing courses,which comprises PE modified bitumen, was reported byDenning et al. [42], although it led to phase separationproblems and higher manufacturing and compactingtemperatures. During the following several years, moreinvestigations [43,44] on PE modified bitumen were beingpublished. Bowering [45] reviewed the necessity ofmodifying bitumen with polymers in 1984 and claimedthat the relatively high cost of PMB might be outweighedby the effects of reduced layer thickness and extendedlife of PMB pavements. In 1987, the US Congress estab-lished the Strategic Highway Research Program (SHRP)which promoted the popularity of PMB by developing aperformance-based specification for both conventionaland modified bitumen with an emphasis on rheology. In1989, Reese et al. [46] reported the good resistance toageing and cracking of PMB after a two-year field test inCalifornia, although they pointed out that further evalua-tions needed to be performed to be conclusive about thesuccess of the modification.

By the early 1990s, increased interest in research ofbitumen polymer modification was observed in manycountries [27]. Researchers systematically investigatedthe mechanical properties, rheology, temperature sensitiv-ity, morphology, thermal behavior, storage stability andageing of different PMBs [48–63]. Both the advantagesand disadvantages of widely used PMBs were graduallyfound out. On the one hand, it was concluded that polymermodification resulted in some improved properties ofbitumen, such as better elastic recovery, higher crackingresistance at low temperatures and higher rutting resis-tance at high temperatures of SBS modified bitumen [58–60]. On the other hand, some drawbacks were proven, suchas the thermal instability of some polymer modifiers andphase separation problems of some PMBs [48,61]. In June

J. Zhu et al. / European Polymer Journal 54 (2014) 18–38 21

1998, a World Road Association (PIARC) International Sym-posium on polymer modification of bitumen was held inRome, which gave an overview of the situation at that timeand encouraged the publication of a report in 1999 [34].Furthermore, attempts to remove PMB’s drawbacks beganfrom the 1990s. In 1996, Giavarini et al. [7] claimed that PPmodified bitumen could be stabilized by adding polyphos-phoric acid (PPA) and they believed PPA could help to im-prove storage stability of PMB by changing the bitumenstructure from sol to gel.

After 2000, investigations regarding PMB tended to bedivided into two fields: (1) continuing to deeply investi-gate the mechanism of polymer modification and its failureand (2) attempting to overcome the disadvantages of somePMBs. The first field mainly focused on the microstructure,deformation, cracking, ageing and fatigue of PMB [64–77].Even now, there are still some academic debates in thisfield. For example, some researchers believe that bitumenhas a heterogeneous colloid structure and PMB should beinvestigated as a multiphase (polymers/asphaltenes/maltenes) viscoelastic emulsion [78,79], shown as Fig. 1;while some other researchers claim that bitumen is ahomogeneous and continuous molecular solution based

Fig. 1. Schematic illustration of the colloidal structure of bitumen and the effecwith increased asphaltenes content in the matrix. (C) Asphaltenes micelles. Ada

on their mutual solubility and polymers result in good ef-fects on PMB by their partial solubility in bitumen [80],seen in Fig. 2. Another example is some authors thinkasphaltenes are strongly polar components in bitumenand the polarity of polymer modifiers has a significantinfluence on their compatibility with bitumen and the finalstorage stability of the resulting PMBs [2,78]; but someothers believe asphaltenes are typical non-polar moleculesfrom a chemical point of view [81]. As for the attempts toovercome disadvantages in 2000s, various ways were re-ported to remove PMB’s drawbacks, including sulfur vulca-nization [82–87], adding antioxidants [22,88,89], usinghydrophobic clay minerals [90–98] and functionalization(including application of reactive polymers) [10,99–113].All these methods will be further discussed in thispaper.

3. Popular polymers for bitumen modification

As mentioned in the above, after World War II ended,synthetic polymers began to be used to modify bitumen.Over the years, researchers developed various polymer

t of polymer modification. (A) Base bitumen. (B) The corresponding PMBpted from [78] with permission from Elsevier.

Fig. 2. (A) The solubility spheres of maltenes and asphaltenes separated from a Venezuelan bitumen. (B) The Hansen solubility parameters of SBS and theVenezuelan bitumen. Adapted from [80] with permission from American Chemical Society.

22 J. Zhu et al. / European Polymer Journal 54 (2014) 18–38

modifiers. Today, widely used polymers for bitumen mod-ification can be classified into two categories: plastomersand thermoplastic elastomers. As Stroup-Gardiner et al.[114] reported, plastomers have little or no elastic compo-nent, usually resulting in their quick early strength underload and the following permanent deformation or brittlefailure. As for thermoplastic elastomers, they soften onheating, harden on cooling [27] and are able to resist per-manent deformation by stretching under load and elasti-cally recovering once the load is removed [114], whichleads to their greater success than plastomers as bitumenmodifiers. Some popular polymers for bitumen modifica-tion are listed in Table 1 with their advantages and disad-vantages. Among them, SBS attracted the most attentiondue to its relatively good dispersibility (or appropriatesolubility) in bitumen as well as the relatively excellentproperties and acceptable cost of SBS modified bitumen[5,115]. Of course, besides these listed polymers, someothers like styrene–butadiene rubber (SBR, randomcopolymers), styrene–butadiene diblock copolymers (SB)and ethylene–propylene–diene monomer rubber (EPDM)were also popular for bitumen modification [116–118]. Inaddition, some small-molecule organic materials, such asPPA and paraffin wax, were also widely used as modifiers

Table 1Popular polymers for bitumen modification [2,4–12,20,23,24,27–29,114,118–125]

Categories Examples Advantages

Plastomers � Polyethylene (PE)� Polypropylene (PP)

� Good high-tem� Relatively low

� Ethylene–vinyl acetate (EVA)� Ethylene–butyl acrylate (EBA)

� Relatively goo� High resistanc

Thermoplasticelastomers

� Styrene–butadiene–styrene (SBS)� Styrene–isoprene–styrene (SIS)

� Increased stiff� Reduced temp� Improved elas

� Styrene–ethylene/butylene–styrene (SEBS)

� High resistancultraviolet

for bitumen. Since they are not typical polymers, they arenot discussed in this paper.

Before reviewing the popular polymer modifiers, it isworth to note that even for a given polymer modifier,selection of base bitumen still has some important effectson the resulting PMB, as each bitumen has its own partic-ular chemical composition and structure. Additionally,base bitumen usually composes over 90% of the PMB byweight, which could introduce overriding influences onthe final properties of the PMB. Good-quality base bitu-men helps to enhance the effects of polymer modification,while poor-quality one may make the modification futile.Regarding the compatibility between polymer and bitu-men, selection of base bitumen is usually completed bylaboratory experiments. However, some theoretical trendswere also highlighted based on the SARA (saturates,aromatics, resins and asphaltenes) fractions of bitumen:for example, high asphaltenes content may decrease thecompatibility between polymer and bitumen and thearomaticity of the maltenes needs to fall between certainvalues to reach a good level of compatibility [78]. Someother researchers even gave the components distributionof base bitumen with the optimum compatibility withSBS [23].

.

Disadvantages

perature propertiescost

� Limited improvement in elasticity� Phase separation problems

d storage stabilitye to rutting

� Limited improvement in elastic recovery� Limited enhancement in low-temperature

properties

nesserature sensitivitytic response

� Compatibility problems in some bitumen� Low resistance to heat, oxidation and

ultraviolet� Relatively high cost

e to heat, oxidation and � Storage instability problems� Relatively reduced elasticity� High cost

J. Zhu et al. / European Polymer Journal 54 (2014) 18–38 23

3.1. Plastomers

As an important category of plastomers, polyolefin is oneof the earliest used modifiers for bitumen. Various polyole-fin materials, including high-density polyethylene (HDPE),low-density polyethylene (LDPE), linear low-density poly-ethylene (LLDPE), IPP and APP [1,6,32,99,126,127], havebeen studied for application in bitumen modification dueto the relatively low cost and the benefits they might bring.Typical Structures of the popular PE and PP are given inFig. 3. After polyolefin materials are added into bitumen,they are usually swollen by the light components of bitu-men and a biphasic structure is formed with a polyolefinphase (dispersed phase) in the bitumen matrix (continuousphase) [119]. As the polyolefin concentration increases,phase inversion occurs in the modified bitumen. Two inter-locked continuous phases are ideal for polyolefin modifiedbitumen, which could improve the properties of bitumento some extent. Those used materials were usually foundto result in high stiffness and good rutting resistance ofmodified bitumen [6], although they are quite different inchemical structure and properties.

However, those used polyolefin materials failed to sig-nificantly improve the elasticity of bitumen [27]. In addi-tion to this, the regular long chains of those polyolefinmaterials give them the high tendency to pack closelyand crystallize, which could lead to a lack of interaction be-tween bitumen and polyolefin and result in the instabilityof the modified bitumen. Furthermore, some researchersclaimed that the compatibility of polyolefin with bitumenis very poor because of the non-polar nature of those usedmaterials [2]. As a result, the limited improvement in elas-ticity and potential storage stability problems of polyolefinmodified bitumen restrict the application of polyolefinmaterials as a bitumen modifier, whereas they are popularin production of impermeable membranes.

More used plastomers in bitumen modification are eth-ylene copolymers, such as EVA and EBA [8,9]. Due to theirsimilar chemical structures, EVA is discussed here as anexample of ethylene copolymers. As seen in Fig. 4, EVAcopolymers are composed of ethylene–vinyl acetate ran-dom chains. Compared with PE, the presence of polar ace-tate groups as short branches in EVA disrupts the closelypacked crystalline microstructure of the ethylene-rich seg-ments, reduces the degree of crystallization and increasesthe polarity of the polymer, which were both believed tobe beneficial to improving the storage stability of modifiedbitumen by some researchers [2]. However, the propertiesof EVA copolymers are closely related to the vinyl acetatecontent. When the vinyl acetate content is low, the degree

Fig. 3. Structures of polyethylene (PE) and polypropylene (PP).

of crystallization is high and the properties of EVA arequite similar to those of LDPE. As the vinyl acetate contentincreases, EVA tends to present a biphasic microstructurewith a stiff PE-like crystalline phase and a rubbery vinylacetate-rich amorphous phase [1]. The higher the vinylacetate content, the higher the proportion of amorphousphase. But the degree of crystallization should be con-trolled carefully when EVA is used as a bitumen modifier,because neither too low (getting easy to be disrupted)nor too high (causing the lack of interactions withbitumen) degree of crystallization is good for bitumenmodification [2].

After EVA copolymers are added into bitumen, the lightcomponents of bitumen usually swell the copolymers. Atlow EVA concentrations, a dispersed EVA-rich phase canbe observed within a continuous bitumen-rich phase[103]. As the EVA concentration increases, phase inversionoccurs in modified bitumen and the EVA-rich phase be-comes a continuous phase. The process of phase inversionin EVA modified bitumen was presented by fluorescentimages as Fig. 5 [123]. If two interlocked continuousphases form in the modified bitumen, the properties ofbitumen could be improved to a large extent. EVA wasfound to form a tough and rigid network in modified bitu-men to resist deformation [9], which means that EVAmodified bitumen has an improved resistance to ruttingat high temperatures.

Although some properties of bitumen are enhanced byEVA modification, there are still some problems limitingits application. One large limitation is the fact that EVAcannot much improve the elastic recovery of bitumendue to the plastomer nature of EVA [4,27]. Furthermore,the glass transition temperature (Tg) of EVA copolymers,which strongly depends on the vinyl acetate content[128], is not low enough to significantly improve thelow-temperature properties of bitumen. It was reportedthat Tg of EVA copolymers with 28.4 wt% of vinyl acetateis �19.9 �C [129], which is even quite close to Tg of somebase bitumen. As a result, EVA’s ability to improve thelow-temperature properties of bitumen is rather limited,especially at high EVA concentrations. According to the re-search by Ameri et al. [121], bitumen’s resistance to low-temperature cracking was increased to some extent byaddition of 2 wt% or 4 wt% of EVA, while the resistance tolow-temperature cracking was decreased when adding6 wt%. In contrast, although EBA could cause potential

Fig. 4. Structure of ethylene–vinyl acetate (EVA).

Fig. 5. Fluorescent images of EVA modified bitumen with various contents (by weight) of EVA. Reprinted from [123] with permission from Elsevier.

24 J. Zhu et al. / European Polymer Journal 54 (2014) 18–38

storage instability of modified bitumen [130], its Tg ismuch lower than that of EVA with the same content ofco-monomer (vinyl acetate or butyl acrylate). It was re-ported that Tg of EBA copolymers with 33.9 wt% of butylacrylate is �45.9 �C, which led to the higher cracking resis-tance of EBA modified bitumen at low temperatures [129].Additionally, the melting temperature of ethylene-richsegments in EVA copolymers is much lower than the usualpreparing temperature of modified bitumen. Those rigidcrystalline domains could be partially broken by the ap-plied shear forces during the preparation [2]. In order toprepare the ideally modified bitumen by EVA copolymers,Airey [123] suggested the upper temperature limit asabout 55 �C. Even so, those ethylene-rich segments stillcould melt and be partially broken by shear when EVAmodified bitumen is mixed with mineral aggregates beforepaving, because the usual mixing temperature is also muchhigher than the melting temperature of ethylene-richsegments.

3.2. Thermoplastic elastomers

Thermoplastic elastomers are usually more effectivethan plastomers for bitumen modification. The most popu-lar thermoplastic elastomers as bitumen modifiers are SBScopolymers and SIS copolymers. Due to their similar chem-ical structures, SBS is discussed here as an example of ther-moplastic elastomers. SBS copolymers are composed ofstyrene–butadiene–styrene triblock chains with a biphasicmorphology of rigid polystyrene (PS) domains (dispersedphase) in the flexible polybutadiene (PB) matrix (continu-ous phase) [2,5], shown as Fig. 6A. The chemical linkagesbetween PS and PB blocks can immobilize domains in thematrix. Tg of PS blocks is around 95 �C and Tg of PB blocksis around �80 �C [103]. Under the usual service tempera-tures of paving bitumen, PS blocks are glassy and contributeto the strength of SBS while PB blocks are rubbery and offerthe elasticity [131]. Furthermore, the incompatibilitybetween PS and PB blocks makes it possible to physicallycrosslink PS blocks as uniformly distributed domains byintermolecular forces at ambient temperatures. This

aggregation of PS blocks disappears at high temperatureswhen the kinetic energy of molecular thermodynamicmovements is greater than the energy of intermolecularforces [132]. However, as shown in Fig. 6, the physical cross-linking among PS blocks can be reformed and the strengthand elasticity of SBS can be restored after cooling, which isvery important for SBS to be a popular bitumen modifier.

After SBS copolymers are added into bitumen, someinteractions happen between bitumen and SBS. Massonet al. [133] reported that intermolecular interactions be-tween bitumen and the PB blocks are stronger than thosewith the PS blocks. They believed that PB blocks interactwith positively charged groups in bitumen through theirp-electrons, whereas PS blocks interact with electron-richgroups in bitumen through their aromatic protons. Mixedwith bitumen, PS blocks in SBS copolymers absorb somesaturated branches and a few rings in light componentsof bitumen [115,124], which leads to the swelling of PSblocks and the hardening of bitumen. When the polymercontent is low, SBS is dispersed as a discrete phase in thebitumen [115]. As the SBS concentration increases, phaseinversion starts in the modified bitumen. The process ofphase inversion in SBS modified bitumen was presentedby fluorescent images as Fig. 7 [9]. It is ideal to form twointerlocked continuous phases: bitumen-rich phase andSBS-rich phase. Within the SBS-rich phase, there are twosubphases: swollen PB matrix and essentially pure PSdomains [115]. Once the SBS-rich phase forms, a rubberysupporting network is created in the modified bitumen,which results in the increased complex modulus and vis-cosity, improved elastic response and enhanced crackingresistance at low temperatures of SBS modified bitumen.

The repeatedly reported excellent properties, relativelygood dispersibility (or appropriate solubility) in bitumenand acceptable cost have made SBS popular as a bitumenmodifier [5,115]. However, SBS copolymers are far fromperfect. For example, the compatibility between bitumenand SBS is not that good [23,83,134]. Storage instability ofSBS modified bitumen was reported with images as Fig. 8[83]. Airey [124] claimed that thermoplastic elastomersand asphaltenes compete to absorb the light components

Fig. 6. Structure of styrene–butadiene–styrene (SBS) and schematic illustration of reversible crosslinks in SBS.

J. Zhu et al. / European Polymer Journal 54 (2014) 18–38 25

of bitumen in SBS–bitumen blends. If these light compo-nents are insufficient, phase separation could occur inmodified bitumen. It was noted that bitumen with higharomatics content can be helpful in producing a compatibleand stable SBS modified bitumen [41] and addition ofaromatic oils can improve the compatibility between SBSand some bitumen with low aromatics content [133]. Toohigh aromatics content in modified bitumen, however,may lead to the swelling and anti-plasticization of somePS blocks [135], which is not good for the resulting proper-ties of the modified bitumen.

Another problem with SBS modification of bitumen isits low resistance to heat, oxidation and ultraviolet (UV)because of the presence of double bonds and a-H in PBblocks [88,136]. In fact, the instability of SBS copolymersis mainly due to the high activity of a-H and low bond en-ergy of the p-bond in double bonds. Undesired chemicalreactions (e.g. formation of peroxy radicals and hydroper-oxides [22]) make them sensitive to heat, oxidation andUV. In order to overcome this disadvantage, researchersfirstly paid much attention to saturated thermoplasticelastomers such as SEBS. A representative patent wasgranted to Gelles et al. of Shell Oil Company [137].

SEBS copolymers, which can be obtained by hydrogena-tion of SBS, consist of triblock styrene–ethylene/butylene–styrene chains. The chemical saturation makes them highlyresistant to heat, oxidation and UV. However, as the doublebonds disappear, some researchers claimed that the polar-ity of the copolymers is considerably reduced [2]. Mean-while, the ethylene/butylene blocks in SEBS have a trendto crystalize [138]. So the compatibility between SEBSand bitumen was believed to become even worse. Accord-ing to the research by Polacco et al. [12], stable SEBS

modified bitumen can only be prepared at a low polymercontent (below about 4 wt% of the total mass) when SEBSacts just as filler and does not improve the viscoelasticproperties of bitumen significantly. On the contrary, whenSEBS content is high enough to really modify bitumen, theprepared PMB is unstable and tends to phase separate.Additionally, extra cost involved by the hydrogenation pro-cess and poorer elastic properties were observed in SEBSmodified bitumen [2], which further limits its applicationas a bitumen modifier.

In order to avoid drawbacks of SEBS modification,researchers from Mexico [138] attempted to use partiallysaturated SBS copolymers in bitumen modification. Theyprepared styrene–butadiene–ethylene/butylene–styrene(SBEBS) copolymers with various degrees of saturation bypartial hydrogenation of SBS copolymers and found thatSBEBS modified bitumen has better mechanical properties(e.g. higher rutting resistance and better elasticity) thanSBS modified bitumen. Although partial hydrogenationmay also cause weaker polarity and possible crystallizationof the copolymers, it was claimed that SBEBS dispersedbetter in bitumen and led to improved storage stabilityof modified bitumen in the research. An explanation forthis phenomenon was given in terms of solubility parame-ters of copolymers in aromatic compounds [138]. However,no further reports on the application of SBEBS are found tosupport its success in bitumen modification.

Another attempt for enhancing the ageing resistance ofSBS modified bitumen was to transfer the double bondsfrom the backbone to branches, i.e. using high vinyl con-tent SBS copolymers. From 1,3-butadiene, people usuallyprepare SBS copolymers with the structure as Fig. 6A by1,4-addition mechanism. Some researchers [139] claimed

Fig. 7. Fluorescent images of SBS modified bitumen with various contents (by weight) of SBS. Reprinted from [9] with permission from Elsevier.

26 J. Zhu et al. / European Polymer Journal 54 (2014) 18–38

that a novel class of SBS copolymers, called high vinyl con-tent SBS copolymers, can be obtained from 1,3-butadieneby 1,2-addition mechanism with special additives and pro-cessing conditions. This SBS copolymer has the doublebonds on the branches, which was believed to result inlower viscosity and better compatibility with bitumen[139]. As heat, oxidation and UV preferably attack doublebonds on branches, the backbone tends to be left intact.So it was claimed that the ageing resistance of SBS modi-fied bitumen modified could be improved by using high vi-nyl content SBS copolymers [139]. In addition to this, whenemployed to modify hard bitumen for base layers, this SBScopolymer was believed to reduce the layer thickness by asmuch as 40% and material cost by some 25% [139]. A rep-resentative patent, which relates high vinyl content di-block copolymers, linear triblock copolymers, multiarmcoupled block copolymers and mixtures thereof, wasgranted to Scholten and Vonk of Kraton Polymers [140].However, people currently do not have much experience

with high vinyl content SBS copolymers. It is still necessaryto carry out more research and field tests to find out towhat extend they work for bitumen modification, espe-cially in service. Care should still be taken now when intro-ducing high vinyl content SBS copolymers to bitumenmodification.

4. Technical developments for removing drawbacks

Although great advances have been achieved in the fieldof bitumen polymer modification, as discussed in the previ-ous sections, there are still various drawbacks which arelimiting its future developments, such as higher costs, somePMBs’ low ageing resistance and poor storage stability.Researchers have attempted different ways to remove thesedrawbacks, including sulfur vulcanization [82–87], addingantioxidants [22,88,89], using hydrophobic clay minerals[90–98] and functionalization (including application of

Fig. 8. Morphology development with the storage time of a SBS modified bitumen at 160 �C. Reprinted from [83] with permission from Elsevier.

J. Zhu et al. / European Polymer Journal 54 (2014) 18–38 27

reactive polymers) [10,99–113]. Including saturation[2,12,138], which has been discussed above, someattempted measurements for removing PMB’s drawbacksare listed in Table 2 with their advantages and disadvan-tages. In the following these are further explained.

4.1. Sulfur vulcanization

Sulfur vulcanization, a chemical process widely used inthe rubber industry, was found to be able to improve thestorage stability of some PMBs with unsaturated polymermodifiers (e.g. SBS modified bitumen) [82–87]. It is be-lieved that sulfur works in two ways: chemically crosslink-ing the polymer molecules and chemically couplingpolymer and bitumen through sulfide and/or polysulfidebonds [83]. These chemical interactions are much strongerthan the physical ones (e.g. the aggregation of PS blocks inSBS copolymers) and they do not disappear even at quitehigh temperatures, which was believed to be very benefi-cial for improving the storage stability of PMB. The cross-linking of polymer molecules leads to the formation of astable polymer network in bitumen; while the coupling be-tween polymer and bitumen directly reduces the possibil-ity of separation.

Although the exact reaction mechanism of PMB sulfurvulcanization is still somewhat unclear, research on rubbersulfur vulcanization and sulfur extended bitumen (SEB)

may be helpful to understand the chemical reactions dur-ing PMB sulfur vulcanization. In the case of SBS modifiedbitumen, addition to double bonds and substitution ofallylic hydrogen atoms could be the main reactions forlinking sulfur and SBS copolymers [142,143]. During thisprocess, the loss of unsaturation, the shift of the doublebonds and a molecular isomerization may occur[142,144]. As for the linkages between sulfur and bitumen,the dehydrogenation of bitumen components and combi-nation of sulfur radicals are possible reactions [145–147].However, due to the complex composition of PMB andthe absence of catalysts (e.g. accelerators and activators)in PMB sulfur vulcanization, all these possible reactionsneed to be critically proven by further studies.

Since the linking of sulfur with polymer modifiers isbased on the chemical reactions with unsaturated bondsin polymer, the application of sulfur vulcanization islimited within PMBs modified by unsaturated polymers,of which SBS is the most widely used one. Sulfur vulcaniza-tion of SBS modified bitumen, on which many patents weregranted, has been industrially used for more than 30 years.It was proven that sulfur vulcanization led to someimproved properties of some PMBs. Besides enhanced stor-age stability, some researchers [83–85,148] claimed thatsulfur vulcanization could also improve the elasticity,deformation resistance and some rheological propertiesof the PMB, but other ones [86,87] found that sulfur

Table 2Attempted measurements for removing PMB’s drawbacks [2,10,12,22,82–113,134,138,141].

Attemptedmeasurements

Advantages Disadvantages

Saturation � Increased resistance to heat, oxidation andultraviolet

� Phase separation problems� High cost

Sulfur vulcanization � Improved storage stability� Good high-temperature properties

� Only applicable for unsaturated polymer modifiers, like SBS� High sensitivity to oxidative ageing and dynamic shear� Hydrogen sulfide released� Poor recyclability

Antioxidants � Reduced oxidation � High cost

Hydrophobic clayminerals

� Improved storage stability� Good rutting resistance� Increased ageing resistance

� Limited improvement in low-temperature properties, ductility andelastic recovery� Hard to be ideally exfoliated

Functionalization � Improved compatibility� More functions not attempted

� Uncontrollability in some cases� High cost

Reactive polymers � Improved compatibility� Enhanced high-temperature properties

� Limited improvement in low-temperature properties� Gelation problems

28 J. Zhu et al. / European Polymer Journal 54 (2014) 18–38

vulcanization made the PMB more susceptible to oxidativeageing and dynamic shear and concluded that it is not agood idea to use sulfur as a sole additional modifier inPMB. Furthermore, hydrogen sulfide, a hazardous gas forboth human health and the environment, could be gener-ated during sulfur vulcanization because of the abstractionof hydrogen atoms in both bitumen and polymer modifi-ers, especially at high temperatures [149–151]. Of course,some researchers might argue that the gaseous emissionis relatively small and most manufacturers know how todeal with the risks and dangers associated. Anotherproblem with sulfur vulcanization is the resulting PMB’spoor recyclability, which might be caused by the chemicalreactions of sulfur during the vulcanization process. Allthese drawbacks are limiting the application of sulfurvulcanization in PMB.

Fig. 9. (A) Layered structure of the 2:1-type clay minerals. Reprinted from [152and exfoliated clay minerals. Reprinted from [153] with permission from Elseviemodified bitumen with hydrophobic clay minerals. Reprinted from [134] with p

4.2. Antioxidants

As previously mentioned, some PMBs are sensitive tooxidation, e.g. SBS modified bitumen. In these cases, usingantioxidants could be helpful to retard oxidation of thePMB. Various antioxidants, including hindered phenols,phosphites and organic zinc compounds, have been intro-duced in PMB in the laboratory. They are believed to workby scavenging the free radicals and/or decomposing thehydroperoxides that are generated in the process of oxida-tion [22,88,89]. These intermediates are very reactive andcontribute a lot to the oxidation. By controlling them, thoseantioxidants were proven to retard oxidation of the PMB tosome extent in the laboratory, but the real service condi-tions of a road are quite different with the laboratory con-ditions. Antioxidants might encounter more problems in

] with permission from Elsevier. (B) Schematic illustration of intercalatedr. (C) The penetration of oxygen in: (a) SBS modified bitumen; and (b) SBSermission from Elsevier.

Fig. 10. Structures of: (A) maleic anhydride (MAH); (B) methacrylic acid;and (C) glycidyl methacrylate (GMA).

J. Zhu et al. / European Polymer Journal 54 (2014) 18–38 29

service, such as their insufficient mobility in the viscousmedium at service temperatures. Authors unfortunatelydid not find any report on field test sections with antioxi-dants in PMB. Additionally, the high cost of introducingantioxidants is also a factor limiting their application[141].

4.3. Hydrophobic clay minerals

Hydrophobic clay minerals have been used in both basebitumen and PMB. It is claimed that their use in PMB ismainly for two aims: (1) improving the ageing resistanceof PMB with barrier properties of the dispersed clay plate-lets and (2) enhancing the storage stability of PMB bydecreasing the density difference between polymer modifi-ers and bitumen [134]. As shown in Fig. 9A, the commonlyused clay minerals in PMB, such as montmorillonite andkaolinite, have a 2:1-type layered structure, which meansthat layers in their crystal structure are made up of two tet-rahedrally coordinated silicon atoms fused to an edge-shared octahedral sheet of either aluminum or magnesiumhydroxide [152]. Every single layer of theirs has a thicknessof around 1 nm [94,95,97,134]. These clay minerals’ abilityto disperse into individual layers at the nanometer leveland to fine-tune their hydrophilic surfaces into hydropho-bic ones through ion exchange reactions [152] makes itpossible to use them in PMB. After mixed, hydrophobic clayminerals disperse in the matrix of PMB. As seen in Fig. 9B,the structure of dispersed hydrophobic clay minerals canbe intercalated or exfoliated [94,153]; and the latter oneis more effective for using in PMB. By adding a proper con-tent of hydrophobic clay minerals, the improved storagestability, increased viscosity, higher stiffness and betterrutting resistance of PMB can be observed [92,93]. Further-more, an improvement in ageing resistance can be achievedby clay platelets’ hindering the penetration of oxygen inPMB [134], which can be presented with Fig. 9C. Excessiveclay minerals, however, may destroy the elastic propertiesof PMB [93]. Additionally, the ideal exfoliated structure ofPMB with hydrophobic clay minerals is hard to obtain;and their use only lead to limited improvements inlow-temperature properties, ductility and elastic recovery[93]. These factors may restrict the application of hydro-phobic clay minerals in PMB.

4.4. Functionalization and reactive polymers

From the view point of bitumen polymer modification,functionalization means the chemical addition of specificfunctional groups to the polymer for obtaining specificfunctions of PMB, such as good storage stability, excellentageing resistance, strong adhesion with aggregates, highstiffness at high temperatures and good cracking resistanceat low temperatures. It is a possible way to overcome thedisadvantages of currently used polymer modifiers andraise the level of bitumen polymer modification in thefuture. By functionalization, various new functions of cur-rently available PMBs may be obtained and even somenew-type polymer modifiers (other than the currentlyused ones) could be developed, for instance reactive poly-mers. In fact, although not typical, saturation also can be

considered as a kind of functionalization, adding hydrogento saturate the polymer.

Although various functions of currently available PMBsmay be obtained by functionalization, most reported inves-tigations mainly aim to improve the compatibility of poly-mer modifiers with bitumen. The added functional groupsare usually expected to interact with some components ofbitumen in various ways such as forming hydrogen bondsor chemical bonds, which may improve the compatibilityto some extent. For instance, Wang et al. [101] preparedfunctionalized SBS copolymers by respectively adding ami-no and carboxylic acid groups during synthesis and claimedthat these functional groups could improve the compatibil-ity of SBS copolymers with bitumen without significantinfluences on their other properties. Meanwhile, otherresearchers functionalized polymer modifiers by grafting.Maleic anhydride (MAH), methacrylic acid and glycidylmethacrylate (GMA), which are structured in Fig. 10, wererespectively attempted to graft some currently used poly-mer modifiers and they were all found to be able to improvethe storage stability of the PMB even with some otherenhanced properties (e.g. higher rutting resistance)[10,99,100,102,154,155]. Besides compatibilization, a fewattempts [156,157] were also made towards better adhe-sion between PMB and aggregates.

Of course, there are also some issues that can be notedregarding functionalization of currently used polymermodifiers. For example, in the case of improving storagestability, excessive interaction between polymer modifiersand bitumen could destroy the biphasic structure of thePMB and make the products useless [2]. In addition, someresearchers claimed that unsaturated polymer (e.g. SBS) isnot supposed to be functionalized by grafting because itprobably causes the undesired crosslinking [2], althoughgrafted SBS copolymers have been prepared and used inbitumen modification by some other researchers[100,158,159].

As for the development of new types of polymer modi-fiers, reactive polymers are examples that cannot bemissed. Reactive polymers used in bitumen modificationare those polymer modifiers which are believed to chemi-cally react (rather than physically mix or interact) withsome components of bitumen [113], e.g. reactive ethylenepolymers and isocyanate-based polymers.

30 J. Zhu et al. / European Polymer Journal 54 (2014) 18–38

Reactive ethylene polymers are mainly reported as eth-ylene-based copolymers containing epoxy rings, e.g. ethyl-ene–glycidyl acrylate (EGA) copolymers and randomterpolymers of ethylene, GMA and an ester group (usuallymethyl, ethyl or butyl acrylate) [2,103,160]. Some of themeven have been used in industry. They are usually claimedto be able to improve the compatibility of polymer withbitumen, as acrylate groups in the molecule are believedto enhance the polymer polarity and the epoxy rings tendto react with some functional groups (e.g. carboxylic acidgroups) in bitumen [2]. However, there are also manyfactors limiting their application. Zanzotto et al. [103] re-ported that bitumen modification with a lower concentra-tion of EGA copolymers did produce high-temperatureproperties similar to modification with a higher concentra-tion of other polymer modifiers (e.g. SBS and EVA) but EGAfailed to improve the low-temperature properties. Accord-ing to the research by Polacco et al. [2], when the contentof reactive ethylene polymers (actually random terpoly-mers of ethylene, GMA and an ester group, called reactiveethylene terpolymers, RET) is high enough to be able toreally modify bitumen, the prepared PMB is unstable andhas a tendency of gelation due to the excessive inter-chainreactions within reactive ethylene polymers. On thecontrary, stable modified bitumen with reactive ethylenepolymers only can be prepared at a low polymer content(usually 1.5–2.5 wt%) when phase inversion does not occurand mechanical properties of bitumen are not improvedsignificantly. It was believed that reactive ethylene poly-mers are not suitable for bitumen modification [2].

About isocyanate-based polymers, they are mainly re-ported as low-molecular-weight polyethylene glycol orpolypropylene glycol (PEG or PPG) functionalized with iso-cyanate groups by reactions with 4,40-diphenylmethanediisocyanate (MDI) [106–113], an example of which canbe seen in Fig. 11. They are claimed to be able to enhancesome mechanical properties of bitumen by chemical reac-tions, mainly at high temperatures. Due to the presence ofisocyanate groups, these polymers were believed to reactwith hydroxyl groups in bitumen [112,113]. When curedwith water, they tend to react with each other to modifythe bitumen at a higher degree [108,112,113]. As a result,isocyanate-based polymers were found to be able to in-crease the viscosity and improve the storage stability andrutting resistance of bitumen at high temperatures[106,113]. But they failed to enhance the low-temperatureproperties as compared with SBS modified bitumen [106].Furthermore, the reactions between isocyanate-basedpolymers may also lead to the gelation risks of modifiedbitumen. Further investigations need to be carried out tosolve the potential problems with bitumen modificationwith isocyanate-based polymers.

Fig. 11. Structure of an isocyanate-based polymer: polyethylene glycol (P

5. Future developments

Ideally speaking, the properties of polymer modifiersshould be very closely designed with the needed PMBcharacteristics, seen in Table 3. After mixed with bitumen,polymer modifiers are supposed to physically or chemi-cally interact with bitumen at a proper degree to form astable biphasic structure with two interlocked continuousphases [53]. As discussed earlier, a low degree of interac-tion between polymer and bitumen could cause a separa-tion problem; while a high degree may lead to thegelation problem and high costs. The interaction betweentwo polymer molecules also should be neither too lownor too high. The polymers with a biphasic structure of adispersed rigid phase in a flexible continuous phase maybe helpful to obtain better PMB properties. With the mod-ification of these ideal polymers, the bitumen propertieswill be improved to a large extent.

In reality, however, it is currently challenging to achieveall the expected properties at the same time. To be practical,compromises will always have to be made and it is thereforeimportant to decide on the dominant characteristics that aremost needed, when designing PMB. As shown in Fig. 12,compromises can be made in two ways: greatly enhancingthe properties with an acceptably high cost or significantlyreducing the cost with relatively poor properties. All previ-ously mentioned efforts for removing PMB’s drawbacks inthis paper, actually, focus on the first compromise.

For enhancing properties, functionalization and thedevelopment of new extra additives are possible direc-tions. Though there are many factors limiting the currentapplication of such functionalized and reactive polymersin bitumen, functionalization does hold a promise for fu-ture development. Hereby it is worth noting that enhanc-ing physical interaction seems easier achievable andcontrollable than chemical interaction when functionaliza-tion is aiming at improved polymer–bitumen compatibilityor PMB-aggregates adhesion. This is due to the fact thatneither bitumen nor aggregates are that chemically reac-tive after artificial refining under very high temperaturesor natural exposure to the environment. Physical interac-tions are thus much easier to achieve and control in thefunctionalization process. Of course, if chemical interac-tions are possible and feasible, they could be more effectivefor enhancing the adhesion between PMB and aggregates.As for developing new extra additives, more effective com-patibilizers, antioxidants and adhesion enhancers could behelpful in improving PMB properties. Regarding compati-bilizers, it is valuable to mention that neither too poornor too great compatibility is good for bitumen modifica-tion, because too poor compatibility causes phase separa-tion problems while too great compatibility only leads to

EG) functionalized with 4,40-diphenylmethane diisocyanate (MDI).

Table 3Designed properties of polymer modifiers with needed PMB characteristics.

Needed PMB characteristics Designed properties of polymer modifiers

To be stiff at high temperatures and soft at low temperatures Low temperature sensitivityTo be adhesive to aggregates Outstanding contributions to adhesion of the resulting PMB with aggregatesTo be workable Excellent dispersibility (or appropriate solubility) in bitumenTo be storage-stable Appropriate compatibility with bitumenTo be durable (ageing- and fatigue-) High thermal stability and stable in-time responseTo be recyclable Strong responsibility for recyclability of the final productsTo be cost-effective Low costTo be environment-friendly Low environmental impact during production and application

Fig. 12. Future developments of polymer for bitumen modification.

J. Zhu et al. / European Polymer Journal 54 (2014) 18–38 31

very limited improvements [53,78]. In the case of enhanc-ing properties, the cost will definitely be increased. Soallowing the degree of enhancing properties to be high en-ough to cover the additional cost will result in more costeffective PMB.

For reducing cost, some cheap polymeric materials,especially wastes and by-products (e.g. waste rubber,waste plastics and polymeric biomass by-products), couldhave potential applications with greater success in the fu-ture. The multifold of research focusing on this domain[161–177] further emphasizes this potential. In spite ofgood environment-friendliness, these wastes or by-prod-ucts usually make some properties of the PMB relativelypoor. So their life costs must be analyzed and proven tobe effective before application. Additionally, waste materi-als usually have their own specific application regimes (e.g.specific climates, specific traffic volume levels) underwhich they perform better than under others. It is morecost-effective to use them under their own specific appli-cation regime, which sounds quite obvious but may be ig-nored or forgotten in the process.

Furthermore, combinations of the two compromises(i.e. using functionalized wastes or using wastes with extraadditives) also could result in acceptable new products.Some research, actually, has started taking this path re-cently and several articles have been published, summa-rized in Table 4. Though all of these claimed someimproved properties, care must be taken with these at-tempts, as they are all isolated research projects and fur-ther investigations still need to be performed to find outwhether they are feasible or not under generic conditions.

Besides the need to compromise between enhancedproperties and costs, several additional points can also be

taken into consideration in future research on bitumenpolymer modification:

(1) Enhancing adhesion from polymer modifiers

Traditionally anti-stripping agents, such as hydratedlime, cement and amines [183–187], have been added toenhance the adhesion of bitumen with aggregates. Silanecoupling agents and sulfur based additives were also usedto help anti-stripping [188–190]. Polymers, however, havethe advantage of utilizing the desirable properties of differ-ent functional groups in the same molecule [191] and havethe possibility to help enhancing aggregates adhesion.Although some of the ordinary polymer modifiers (e.g.SBS and EVA) were also reported to lead to improvementsin adhesion [15], none of them were specially designed forenhancing adhesion and their capability to help anti-strip-ping is quite limited. It has long been believed as a prom-ising strategy to use specially designed polymers forenhancing adhesion between bitumen and aggregate[191]. Using extra polymeric adhesion enhancers and com-bining the function of enhancing adhesion with polymermodifiers are both possible directions, but the latter oneis definitely more efficient. In fact, some efforts have beenmade in this direction. For example, Crossley et al.[157,192] specially designed and prepared functional poly-isoprene modifiers with amino or silane groups at one endof the polymer chain to improve the adhesion of bitumenwith aggregates. It was found that high-molecular-weightsilane-functional polyisoprene, which was essentially apolymeric silane coupling agent, helped enhancing boththe moisture resistance and low-temperature properties

Table 4Trials towards combining enhancing properties and reducing costs in PMB.

Attempted combinations Conclusions ReferenceNo.

Grafting of waste plastics with maleic anhydride(MAH)

MAH grafting significantly improved the storage stability of bitumen modified withwaste plastics

[178]

Combination of polyethylene (PE) packagingwastes with hydrophobic clay minerals

Hydrophobic clay minerals improved the low-temperature properties of modifiedbitumen without adverse influence on high-temperature properties

[179,180]

Grafting of eucommia ulmoides gum (EUG)* withMAH

An appropriate amount of grafted EUG can enhance both the high- and low-temperature properties of styrene–butadiene–styrene (SBS) modified bitumen, inspite of the currently high cost

[181]

Synthesis of pre-polymers** with castor oil*** and4,40-diphenylmethane diisocyanate (MDI)

Modification with the pre-polymers enhanced the rutting resistance of bitumenwith much lower producing temperature and higher thermal stability thanordinary polymers

[182]

* EUG, a natural trans-polyisoprene from eucommia trees.** This attempt synthesized pre-polymers rather than typical polymers.*** Castor oil, a natural triglyceride from castor seeds.

Table 5Investigations on the actual long-term performance of field test sections with PMB.

Yearinvestigated

Location ofsites

Amountof sites

Polymer information In-servicetime beforeinvestigated

Conclusions ReferenceNo.

1990 USA,Canada andAustria

Morethan 30

Various polymers includingPE, EVA, SBR and SBS

Less than5 years

� No significant difference was observed inperformance between most test sections and thecontrol ones

[193]

1993 USA 6 Various polymers includingEVA, SBR and SBS

Various, nolonger than73 months

� No distinctive pattern was found between theperformance of modified and unmodifiedbitumen, nor among the performance of thesame modified bitumen types, when comparedbetween different sections

[51]

1995 USA andCanada

20 Various polymers includingLDPE, some unspecifiedpolyolefin, EVA, SBR andSBS

Various, nolonger than9 years

The lack of related information made it differentto draw more than a couple of specificobservations:� EVA modification has a tendency for brittlebehavior as seen by the reports of prematurecracking; and� There were no consistent trends in ruttingresistance for any of the reported modifiers

[114]

2002 USA 1 Various polymers includingLDPE, SBR and somestyrene–butadiene blockcopolymers

11 years � For most test sections, the use of PMB didimprove the field cracking resistance over theunmodified bitumen. However, LDPE increasedthe brittleness of the bitumen and mixture,leading to extensive cracking� Bitumen modification is not necessary tocontrol rutting. Properly designed andconstructed mixture can perform under heavytraffic without rutting

[194]

2007 Switzerland 16 Various polymers includingPE, EVA, SBS and EPDM

19 years � After 14 years, PMBs showed some improvedperformance. Especially, one section with SBSmodified bitumen showed great crackingresistance. However, one section with basebitumen performed as well as some PMBs� After 19 years, the crosslinked polymermodified bitumen showed very good durability

[197,198]

2011 Canada 7 Various polymers includingSBS, SB and RET

8 years � Bitumen modified with RET and PPA performedas desired, without virtual crack after eight yearsof service� One of the two SBS modified bitumen sectionscracked at a moderate amount, with intermittentfull width transverse cracks of moderate severity� The remaining sections all experienced severeand excessive distress, with numerouslongitudinal and transverse cracks

[199–201]

32 J. Zhu et al. / European Polymer Journal 54 (2014) 18–38

J. Zhu et al. / European Polymer Journal 54 (2014) 18–38 33

of the mixture. More other attempts are supposed to becarried out in this direction.

(2) Long-term performance of PMB

PMB is expected to perform well in the field over a longtime. To evaluate and improve this long-term performance,much research has been performed [51,114,193–201]. Onefocus area in this was testing the actual field performanceof newly-developed products by placing and monitoringfield test sections [114]. In the late 1980s when the appli-cation of PMB started to be promoted by SHRP, many fieldtest sections with PMB were constructed; and severalinvestigations on the actual long-term performance wereconducted in the following years, seen in Table 5, althoughsome of them also paid attention to some other additives.Unfortunately, no consistency was found between theseinvestigations but one: there is not much regularity ob-served by these field test sections due to the short in-ser-vice time and various uncontrollable factors in field. Theother main focus area has been the measuring of durability[195–196,202] by laboratory accelerated tests, such as roll-ing thin film oven tests (RTFOT) and pressure aging vesseltests (PAV). Fundamental properties of PMB, like stiffnessand shear complex modulus, were considered to be moreindicative than empirical ones [51]. However, the relation-ship between these laboratory results and the actual fieldperformance is still not well understood. It could also beargued that the currently performed laboratory oxidativeageing protocols fail to replicate the oxidative aging thatoccurs in the field, which other researchers have also men-tioned [200,201]. Today, the long-term performance ofPMB, both from an economical and environmental per-spective, is becoming more important. So in future re-search, whether investigating currently available PMBs ordeveloping new-type polymer modifiers, the long-termperformance of the PMB should be a major consideration.

(3) Recyclability of PMB

Almost 30 years have passed since PMB began to beincreasingly used in the late 1980s. Many of the early-constructed PMB pavements have reached the end oftheir service life and need resurfacing [203]. It complieswith the principle of sustainable development to recyclePMB after its service life ends. Researchers tried to inves-tigate the recyclability of PMB, especially the mostwidely-used SBS modified bitumen [203–209]. Althoughsome of these investigations concluded that it is techni-cally feasible to recycle aged PMB by adding rejuvenatorsor virgin bitumen [203–206], there is still no widely-accepted PMB recycling technique available today, whichalso affects the popularization of PMB in turn. Addition-ally, the mechanism of PMB ageing and rejuvenating isstill not well understood. So in the future, more researchshould be focused in this direction. As for developingnew-type polymer modifiers, the concept of sustainabledesign should be introduced. Many of the current prob-lems with recycling result from the fact that the property

of recyclability was not involved when most productswere designed. If a modifier is initially designed withrecyclability in mind, it will lead to products with betterevaluation of life cycle and its popularization will bemuch easier.

6. Conclusions and recommendations

This paper reviews the achieved advances and encoun-tered challenges in the field of bitumen polymer modifica-tion during the last 40 years. The largely discussedtechnical developments include the application of somepopular plastomers (PE, PP, EVA and EBA) and thermoplas-tic elastomers (SBS, SIS and SEBS), saturation, sulfur vulca-nization, adding antioxidants, using hydrophobic clayminerals and functionalization (including application ofreactive polymers). Based on this overview, needed futuredevelopments of polymer for bitumen modification wereanalyzed and the following conclusions and recommenda-tions are drawn:

(1) Polymer modification has been proven to be aneffective way to improve bitumen properties tosome extent by many researchers and has been usedwidely in practice. However, the currently popularpolymer modifiers have various disadvantages limit-ing their application. Some important problems withbitumen polymer modification are still not wellunderstood. More efforts are supposed to be madeto promote a further development.

(2) Researchers tried various solutions to remove draw-backs of currently used polymer modifiers, amongwhich saturation, functionalization (including appli-cation of reactive polymers) and using extra addi-tives (sulfur, antioxidants and hydrophobic clayminerals). These solutions do overcome some disad-vantages of PMB, but most cause some new prob-lems. So more research needs to be carried out inthe future to solve these problems and find newways to modify bitumen effectively and cheaply.

(3) Since it is currently challenging to perfectly achieveall expected PMB properties at the same time, somecompromised ways might be optional for the futuredevelopment of bitumen polymer modification:greatly enhancing the properties with an acceptablyhigh cost, significantly reducing the cost with rela-tively poor properties or their combinations. Func-tionalization is considered as a promising way toenhance the properties of currently used polymersand develop new-type polymer modifiers with muchgreater success in the future.

(4) It is recommended that future research on bitumenpolymer modification pay more attention to the fol-lowing points:

� Function development of enhancing adhesion

with aggregates for polymer modifiers;� Long-term performance of PMB; and� Recyclability of PMB.

34 J. Zhu et al. / European Polymer Journal 54 (2014) 18–38

Acknowledgements

The authors thank Måns Collin and Per Redelius fortheir comments on this paper. Jiqing Zhu gratefullyacknowledges the scholarship from China ScholarshipCouncil.

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