Biomaterials and Their Potentialities as Additives in Bitumen

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This entry is adapted from the peer-reviewed paper 10.3390/molecules27248826

A lot of research is ongoing to improve bitumen’s properties due to its use as a binder in road paving processes. Over the years, the most common method to improve bitumen’s properties has been with the use of additives. The major drawback in the use of these additives is the fact that they are substances of strong chemical nature which are either too acidic, too basic or emit toxic fumes and volatile organic compounds into the environment. In the long run, these chemicals are also toxic to the road pavement personnel that carry out the day to day industrial and paving operations. This led researchers to the initiative of synthesizing and applying biomaterials to be used as additives for bitumen. In this light, several studies have investigated the use of substances such as bio-oils, natural waxes, gum, polysaccharides and natural rubber.

biomaterials bitumen bio-oils polysaccharides additives circular economy

1. Introduction

Research on biomaterials has been pushed to the forefront of scientific investigation in recent years due to concerns about the emission of greenhouse gases (GHG), global warming, demand for renewable resources and high energy consumption in general. The need for a circular economy and environmental sustainability makes it also important to reduce our dependence on non-renewable resources and drift towards alternative bio-renewable resources for industrial purposes and common day to day activities ^[1]^[2]^[3]. Apart from the economic advantage of the use of bio-derived materials which are less costly and reduce energy expenditure, biomaterials are also environmentally friendly as they are biodegradable, renewable and facilitate strategic resource conservation ^[4]^[5]^[6]. Biomaterials are also known to reduce the greenhouse effect via CO₂ conservation ^[7]. All of this goes a long way in promoting a circular economy and eco-sustainability in general. In asphalt technology, biomaterials can be used in different capacities: biomaterials with Warm Mix Asphalt (WMA), biomaterials with Reclaimed Asphalt Pavement (RAP) to rejuvenate aged bitumen, bio-binders as partial or total replacement of petroleum-based binders (bitumen) and so on ^[8].

The application of bio-binders in the asphalt industry differs depending on the aim of the research study in which it is being used. In the real sense, a bio-binder is a substance of biological origin that serves as an adhesive in the asphalt mix. Conventionally, in the asphalt industry, bitumen functions as the binder but as a result of the continuous research in the field of material science, other more bio-degradable bio-binders are being investigated. The experimental application of bio-binders can be classified, thus ^[9]:

(a): Direct alternative (75–100% bitumen substitute);
(b): Bitumen extender (10–75% bitumen substitute);
(c): Bitumen modifier (<10% bitumen substitute).

Although the total substitution of conventional asphalt binder is advantageous and preferable, most bio-based binders do not have the necessary properties to produce high quality pavement and for now cannot be used as complete substitutes for asphalt bitumen ^[10]^[11]^[12]. Research is, however, still going on in this regard. Studies involving biomaterials as bitumen modifiers are the most abundantly available on scientific database due to the fact that there are more researchers working on this subject ^[8]. Herein, biomaterials and their potential to be used as effective additives to improve bitumen binder’s characteristics in asphalt conglomerate as reported in previous studies will be summarized and discussed. This applies to Warm Mix Asphalt (WMA), Reclaimed Asphalt Pavement (RAP), Cold Mix Asphalt and any asphalt technology which entails bitumen modification in order to produce durable asphalt pavements. Several types of biomaterials have been used in recent years as bitumen additives and quite a number of them have yielded positive results by improving specific characteristics of bitumen. Biomaterials are materials that partially or totally constitute a bio-binder. Biomaterials such as natural waxes, bio-oils, biopolymers, organic waste and bio-based nanomaterials have all been reported in literature to improve bitumen’s characteristics ^[13]^[14]^[15]^[16]^[17]^[18]^[19]. All the aforementioned types of biomaterials are obtained from different source materials and processes, although, in some cases, different biomaterials can be obtained from the same process or starting material. For example, bio-oil can be obtained via pyrolysis but so can bio-char which can also be classified as a nanomaterial. Bio-char can also be categorized as organic waste but so can Waste cooking oil, which can also be regarded as a bio-based oil. Figure 1 illustrates the classification of the different classes of biomaterials and their relativity to one another.

Figure 1. Biomaterials used as bitumen additives and their categorical relativity.

The different types of biomaterials have different mechanisms of action in bitumen, and this can go a long way in characterizing different classes of biomaterials as additives for bitumen depending on their effect. Bio-oils, for instance, are used as bitumen fluxers by reducing the viscosity of the bitumen and, in the long term, increase the stiffness of the bituminous asphalt pavement by a process called oxidative polymerization. Natural waxes, below their melting point, increase the stiffness and elasticity of bitumen while, above their melting point, decrease the stiffness and increases bitumen’s viscosity. Biopolymers such as polysaccharides and natural rubber modify bitumen by changing its visco-elastic behaviour by increasing bitumen’s range of plasticity. Nanomaterials, on the other hand, often improve the tensile strength of the asphalt conglomerate by forming strong intermolecular crosslinks in bitumen’s matrix, which in the long term translates to increased stiffness and tensile strength of the paved asphalt ^[9]^[14]^[20]^[21].

From the physico-chemical point of view, a given mechanism of action is the overall result of a delicate equilibrium of all the intermolecular interactions, and therefore dynamic processes, involved: polar and apolar interactions, π-interactions, Van deer Waals and dispersion interactions, eventual H-bonds, etc., all of them exerting their effect with a definite strength and with a definite length-scale and whose combination dictates the complexity of the system ^[22]. In fact, it is clear that the chemistry of any bitumen is the key element to define the physical properties: following the analogy with reversed micelles in water-in-oil microemulsions, where polar organic domains are stabilized and dispersed in a more apolar matrix, ^[23]^[24]. The stabilization of the polar, asphaltene-based domains is of pivotal importance for determining the structure and properties of the overall aggregates, even if the stabilization mechanism must be considered to be quite general, involving, besides organic materials, also inorganic complexes ^[25], inorganic materials ^[26] and even nanoparticles ^[27].

As a consequence of these strict relationships between intermolecular interactions, aggregates structures and their dynamic properties, the rheology (ductility at a given temperature/frequency) and behaviour of bitumen are dependent not only on its structure, but also on the maltene’s glass transition temperature and the effective asphaltene content ^[28]. Bitumen’s mechanical characteristics, as well as their dependence upon time, ageing, presence of additives and/or modifiers, rejuvenation and so on are therefore interpreted as a result of the tendency of consequent rigidity occurring when stronger intermolecular connectivity appears, compared to a quite less dense intermolecular network of virgin bitumen, where, for example, the asphaltene domains are poorly connected to each other. As a result of the aggregation-based processes of modification, a progressive tuning of the viscoelastic properties can be achieved. In this picture, the resin molecules, due to their amphiphilic nature, have a special role which needs to be emphasized. In fact, they tend to reduce the associative interactions between the asphaltene particles by interposing itself between the asphaltenes and the maltenes. Similar mechanisms are expected to be common also in bitumen chemistry: the interaction of the apolar part of the resin (its apolar moiety) with the maltene phase will draw the latter towards more hindered dynamics typical of the stiffened asphaltene-dominated structure. Another mechanism can be envisaged, i.e., the formation of direct interactions between the surfactant polar headgroup and polar parts of asphaltene. In fact, it has been recently highlighted that, in addition to polar and apolar interactions, further specific interactions between surfactants themselves with consequent peculiar self-assembly processes ^[29]^[30] dictating the final overall aggregation pattern ^[31] and the (usually slowed-down) dynamics ^[32]^[33].

2. Bio-Oils

Oils in general are viscous liquids derived from petroleum and are principally used as fuels or lubricants. Due to the diverse nature of oils and the variability of their constituents, for example, fatty acids of differing natures, oils have a broad range of secondary uses and functions. Bio-oils utilize biomass as the starting material instead of petroleum-derivatives. These oils are obtained from the rapid heating of biomass in a vacuum condition ^[34]. The application of bio-oils in material science is very advantageous because it is renewable, eco-friendly and facilitates conservation of resources by its widespread use ^[9]. Bio-oils can be obtained by either thermochemical liquefaction or pyrolysis with the latter method being the more eco-sustainable process since it does not involve heavy thermal cracking or hydrogenation of hydrocarbons ^[35]. Although pyrolysis processes differ slightly in a few parameters and are therefore categorized into, slow, conventional, fast and flash pyrolysis, the commonly used conventional process takes place between 300–600 °C ^[17]^[36].

Bio-oils from pyrolysis of biomass are liquid emulsions of oxygenated organic compounds, polymers and water. These compounds are mixtures of different components such as furfural, phenols, aldehydes, ketones, ethers, esters and so on. Figure 2 shows the chemical groups of compounds derived from lysis of biomass some of which are described later on in this work. Pyrolysis of biomass also produces another biomaterial as a by-product known as bio-char, which is the solid residue obtained alongside bio-oil. Biochar will also be discussed under a different section. Bio oils from pyrolysis are mainly used as bitumen rejuvenators because they contain hydrocarbons which could substitute the hydrocarbons which have been oxidized as a result of bitumen aging ^[37]. Table 1 shows the composition of pyrolysis bio-oils and the percentages of their components. A 2022 study carried out by Caputo et al. ^[17] demonstrated that bio-oil derived from pyrolysis has anti-oxidant properties and is capable of regenerating aged bitumen due to the chemical similarities in the hydrocarbons present in the bio-oil and those present in the aged bitumen which have been depleted by oxidative aging. Their study showed that a 2% modification by weight of bitumen with a pyrolysis oil restored the properties of aged bitumen to a state similar to that of unaged virgin bitumen. The carbonaceous nature of the bio-oil’s hydrocarbons brings about an enhanced rejuvenating effect by replenishing the depleted hydrocarbons in the bitumen via adsorption and other chemical interactions which are still under investigation. The effectiveness of pyrolysis oils in rejuvenating bitumen brings about longer-lasting, easy-to-regenerate asphalt pavements thereby promoting resource conservation towards a circular economy. A 2008 study ^[38] estimated a 23% reduction in energy consumption if asphalt from Reclaimed Asphalt Pavement (RAP) is reused for new paving operations after being rejuvenated by bitumen rejuvenators. In this case, bio-oils derived from pyrolysis can be said to have properties of a bitumen rejuvenator. Not a lot of work has been done on the rejuvenation of RAP bitumen using pyrolysis oils, but it can be projected that, in the next few years, the use of pyrolysis oils in bitumen rejuvenation will be more widespread and researched, thus leading to a better characterization of the exact mechanism of the replenishment of depleted hydrocarbons in aged bitumen.

Figure 2. Chemical groups derived from the breakdown of biomass. Reproduced with permission from Machado et al., Bio oils: The next-generation source of chemicals; published by MDPI Reactions, 2022 ^[39].

3. Biopolymers

The term biopolymer has a broad meaning because it refers to a diverse range of materials in different forms. The characteristic that these materials have in common is that they are polymeric substances which are biodegradable regardless of the form they are in (fibres, powders, gums, pellets and so on). Biodegradability of biopolymers depends on their origin and chemical structure in combination with the environmental degrading conditions ^[40]^[41]. Depending on the mechanism of degradation, biodegradable polymers are divided into two groups, namely:

(a): Synthetic biopolymers (are degraded via hydrolysis or oxidation);
(b): Natural biopolymers (are degraded enzymatically).

3.1. Synthetic Biopolymers

Synthetic biopolymers are mostly biomaterials derived from petroleum sources and are not exactly biopolymers but can be more accurately described as biodegradable polymers. Some are synthetic polymers with hydrolysable backbones, which makes them susceptible to biodegradation under certain conditions. These include polyesters, polyamides, polyurethanes and polyanhydrides ^[42]^[43]. The other type of synthetic biodegradable polymers are conventional polymers to which additives have been added to facilitate their degradation. Classic examples of synthetic biopolymers are Polylactic acid, Polyglycolic acid, Polybutylene succinate and Polycaprolactone ^[44].

3.2. Natural Biopolymers

Natural biopolymers are synthesized in biological systems (plants, animals and microorganisms) or are obtained from biological starting materials which have been synthesized artificially—for example, polysaccharides such as starch and cellulose, natural fats and so on. The biodegradability of these biopolymers is very high with the only factor being the varying time frame required for their complete degradation and decomposition, which ranges from a few days to months or even a couple of years ^[45]. Polysaccharides are a major group of natural biopolymers, with starch and cellulose being the most commonly applied polysaccharides in material science and asphalt technology. Other less common polysaccharides such as alginate and chitosan are also used but to a lesser extent. Natural rubber is another important biopolymer used in asphalt technology and material science in general. Natural rubber is a bio-elastomer which possesses unique characteristics in its pure state or when combined with synthetic elastomers ^[9]^[46]. Lignin, phospholipids and animal fat are other naturally biodegradable substances from which biopolymers for asphalt can be obtained.

4. Waxes

Waxes are lipophilic organic compounds such as lipids and higher alkanes which are insoluble in water but soluble in organic, non-polar solvents. They consist of long chains of aliphatic alkyls and sometimes even aromatic compounds. Depending on the source of the wax, they also contain various functional groups such as fatty acids, aldehydes, alcohols and esters of fatty acids ^[47]. Waxes have a number of industrial applications but are mostly combined with other substances to produce coating formulations and colourants ^[48]. Industrially, synthetic waxes are more widely used, but, recently, due to the need for eco-friendly practices, the demand for the use of natural waxes is increasing. This is due to the biodegradability of natural waxes, which the synthetic waxes such as paraffin wax are not capable of.

In bitumen technology, waxes and wax-based additives are used to lower bitumen viscosity, improve workability and enhance lubrication ^[49]. The reduced viscosity brings about an increased stiffness, which occurs as a result of the solidification of the wax into microscopic particles uniformly distributed in bitumen’s matrix upon its cooling ^[50]. Waxes have melting points below asphalt processing temperatures and thus melt and become dispersible in the mix during asphalt production processes. Since waxes are apolar, they feed the maltene fraction of bitumen, thus improving its flow and workability during asphalt paving processes, and this would better disperse asphaltene clusters at their various levels of aggregation in bitumen’s matrix ^[51]^[52]. In most cases, waxes have little or no effect on bitumen’s rheological properties even though they modify the binder’s viscosity. Caputo et al. ^[53] reported that a waste food wax additive at dosages of 0.4–3% by weight of bitumen improved the workability of asphalt conglomerate acting on the viscosity of the bitumen without changing its rheological properties. Sigwarth et al. in a recent study ^[20] tested the efficiency of 10 bio-degradable waxes and their findings show that the effects of the waxes on bituminous mixes were significantly affected by the wax origin, composition and specific melting point. The waxes when applied in a 3% proportion by weight of bitumen were able to increase the stiffness and elasticity of the bitumen above their melting point temperatures. The results also demonstrated that two of the waxes performed excellently and have the potential to completely replace the synthetic waxes (Sasobit and Licomont), which were compared to the biodegradable waxes in the study. Oliviero-Rossi et al. ^[13] tested the potential of a food-grade phospholipid wax as a bitumen additive, and their results show that the phospholipid at dosages ranging from 1–6% improved bitumen’s rheological properties. They also demonstrated that the phospholipid wax enhanced the adhesion between bitumen and the aggregates in asphalt mix by reducing the interfacial energy between the aggregates and bitumen binder. In general, the applicability of natural waxes as bitumen additives, apart from being eco-friendly, can also foster circular economy via recycling, environmental protection by reducing harmful emissions during production processes and resource conservation since most of these wax-based additives are waste products of other processes and are, most importantly, biodegradable.

5. Nanomaterials

The use of nanomaterials is becoming increasingly popular in the asphalt industry due to the fact that nanotechnology principles can be applied to modify the characteristics of materials at the atomic (nano) level. Macro-properties of materials can be modified at the nano scale. At the nano scale, the material properties are noticeably different compared to the macro scale. This is due to higher surface area to volume ratio at the nano scale and, at such small dimensions, quantum effects are pronounced ^[54]. In general, nanomaterials have particle sizes between 1–100 nm, and this allows for the formation of nanostructures within bitumen’s matrix, thereby conferring some beneficial effects on the bitumen of interest ^[55]. Most nanomaterials used in asphalt improvement are of natural origins with a huge percentage of these materials being mineral substances which are not biodegradable in the real sense. Some of these mineral-based nanomaterials include nanoclay, graphite/graphene and their oxides, nanosilica and carbon nanotubes. Biochar, which is a by-product of pyrolysis of biomass, is also a nanomaterial of interest and can improve the properties of bituminous mixes. A few food-based powder additives are also capable of forming nanostructures within bitumen.

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