# Adhesion Promoters in Bituminous Road Materials: A Review
## 3 Survey of Types of Adhesion Promoters Currently Used in the Asphalt Industry and Their Functions  
The chemical affinity between bitumen and aggregate can be favored by the addition of trace amounts of organic additives, which modify the interface properties of both the aggregate surface and bitumen. These compounds are known as adhesion promoters. The goal is to maximize the chemical-physical interaction between bitumen and mineral surface. The topic regarding the of adhesion promoters designed for road paving industry is strictly connected to the WMA technology, which is characterized by working temperatures (110-140 \({}^{\circ}\)C) lower than the traditional hot mixing [5]. Under those conditions, it is likely that bitumen comes in contact with aggregates with residual moisture, which in the long term is considered to favor undesirable stripping phenomena, which was previously described in the Introduction. Therefore, there is a technological need for the addition of organic compounds to bitumen to increase its coating efficiency onto the stones and promote a better bitumen adhesion. A successful adhesion promoter will be able to impair the intermolecular interactions between residual water and polar aggregate surface and consequently lower the contact energy between the bituminous apolar liquid phase and the solid mineral phase. Figure 1: Adhesion failure at the aggregate-bitumen interface due to the penetration of water is termed “stripping”. In some technical reports, these organic additives are widely classified according to the following two categories, namely, Passive Adhesion Promoters (PAPs) and Active Adhesion Promoters (AAPs) [28]. The main difference refers to the ability of the former, also called anti-strip additives (ASAs), to maintain a strong adhesive bond between binder and dry aggregate surface and to prevent stripping under wet condition once bond formation is achieved, while the latter promote bonding between bitumen and aggregate surface, even if there is residual water present on the surface of the aggregate. Therefore, the last property is less restrictive than the passive adhesion because with AAPs even wet aggregate surface may be coated. The latter class of compounds are basically emulsifier surface active agents whose first application was reported in the literature in the sixties by [PERSON] [29]. They form oriented monolayers at the interface between the apolar bitumen and the aggregate polar surface and act by decreasing the surface tension of bitumen. While the head groups bind strongly to the aggregate surface, the hydrocarbon tails of the molecules are compatible with the binder. AAPs thus act as a bridge between the bitumen and the mineral surface and displace water at the interface (see Figure 2). In some of the patented formulations, the chemical additive contains a surfactant, which acts as lubricant and works at the microscopic interface between the polar mineral aggregate and hydrophobic bitumen [30]. Amines, poly-amines, and amido-amine adhesion promoters (AMAPs) represent one of the most common classes of organic additives, which are basic compounds [31]. The supposed anti-stripping mechanism is supported by the nitrogen electron lone pair, which reacts with acidic silica surfaces, hence enhancing adhesion. Another explanation is linked to the capacity of Lewis bases to capture protons from acidic groups of bituminous polar components, leaving the acids negatively charged. The formation of quaternary ammonium ions (R-NH3+) provides an electrostatic favorable interaction with the aggregate surface, leading to adhesion enhancement. The optimum adhesion power is the result of a compromise between the length of the apolar hydrocarbon chain and the number of amine polar groups. The best performance can be reached with 14-18 carbon chains, linked to one or even two amine groups, with at least one primary amine group. Recently, a type of AMAP has appeared on the market with the tradename Rediset(r) WMX from Akzo Nobel, Amsterdam, the Netherlands [32].