# Adhesion Promoters in Bituminous Road Materials: A Review
## 3 Survey of Types of Adhesion Promoters Currently Used in the Asphalt Industry and Their Functions
This material, formulated in combination with a non-foaming non-ionic additive, contains a fatty polyamine structure with a long chain aliphatic hydrocarbon, which is able to promote both active and passive adhesion [33]. It is a combination of surfactants and organic additives in which the surfactants improve the wetting of the aggregate surfaces with binder by active adhesion, and the other components reduce the binder viscosity [34]. Cecabase(r) from Ceca Arkema Group, La Garenne-Colombes, France, is another powerful chemical additive, which has been found to improve coating of the aggregate particles, workability, and compaction [30]. It is formulated as a combination of emulsification agents, surfactants, polymers, and AP additives [35]. Several investigators reported that addition of Cecabase(r) to bitumen has no noticeable effects on the rheological properties of asphalt binder but improves workability at low mixing temperature, which is a desirable condition for WMA design [36]. Figure 2: Active adhesion is ensured by the favorable orientation of positively charged surface-active molecules by decreasing the contact angle between bitumen and the negatively charged aggregate interface, allowing the water displacement, and maximizing the surface coating. Other compounds such as naphthenates, metal salts of organic acids obtained from the alkali washes of petroleum fractions, have also been demonstrated to play the role of adhesion promoters [37]. It has been presumed that iron naphthenate operates as an ASA by migrating to the aggregate-asphal interface (while the asphalt is still hot) and forming a water-resistant complex [38]. Another group of APs is represented by organosilanes-based adhesion promoters (OSAPs) R\({}_{1}\)Si(OR)\({}_{3}\) possessing a hydrocarbon chain with affinity for bitumen and a polar silane end group with affinity for inorganic surface [39; 40]. [PERSON] et al. [40] have recently shown that the addition of OSAPs can improve the bitumen wettability towards mineral stones at concentrations as low as 0.01 wt. % (Figure 3). Moreover, the adhesive performance of the modified binder has been quantitatively determined by contact angle testing and found to be in excellent agreement with the conventional empirical boiling test [40]. The latter method, ASTM D3625, is a standardized procedure for evaluating the moisture sensitivity of an asphalt-aggregate mixture [41]. Briefly, a certain amount of aggregate sample covered with bitumen is immersed in boiling water for 10 min. Then, the mixture is allowed to cool while the stripped asphalt is skimmed away. The water is drained, and the wet mixture is placed on a paper towel and allowed to dry. Visual rating is conducted to assess the level of stripping. The contact angle approach through the goniometer and the Wilhelmy plate methods is another powerful technique that is able to evaluate the effects of surfactant-based additive on the surface free energy of a modified binder. The technique is reviewed elsewhere, for example, in [35]. Polymer materials have also been found to be efficient as anti-stripping additives. [PERSON] et al. [42] showed that Styrene Butadiene Styrene (SBS) copolymer significantly enhanced moisture resistance of slag aggregates in harsh desert environments. Moreover, bitumen modified with SBS in conjunction with polyphosphoric acid (PPA) positively affected wettability performance and increased binder stability during long-time storage at elevated temperatures [43; 44; 45; 46]. For example, PPA added to bitumen in a few wt. % is responsible for a stiffening effect due to several chemical mechanisms detailed in [44; 47; 48], which overall would promote an increase of the effective volume of asphaltenes, thus enhancing the rigidity of bitumen. The hardening effect played by PPA has also been compared to bitumen containing different commercial types of waxes [49]. An interesting comparison between AMAPs, OSAPs, and SBS has recently been reported by [PERSON] et al.