# Bitumen and Bitumen Modification: A Review on Latest Advances
## 2. Bitumen Polymers
### Natural and Synthetic Rubber  
#### 2.4.1 Natural Rubber  
Natural rubber polymer could be easily synthesized through the chemical reaction of isoprene molecules. The resulting compound, the cis-1,_A_-polyisoprene with a chemical formula of (C\({}_{5}\)H\({}_{8}\))\({}_{n}\) with high molecular weight is analogous to natural rubber [75; 76]. In each repeated unit of polyisoprene, the double bond presence makes the polymer easily undergo a vulcanization process. This consists in the addition of sulfur to change the properties of natural rubber into a thermoset [77]. From a physical point of view, the presence of complex molecular chains of polyisoprene gives natural rubber a large elasticity and resilience characteristics [78]. For practical application of natural rubber, vulcanization is needed. However, in this process rubber creates disulfide bonds between chains, limiting its degree of freedom and the chains speedily tighten. Thus, the rubber is harder and less extensible because of an increasing in elastic force [79].  
Natural rubber latex has been applied in the bitumen industry for over 30 years and is perceived to have potential for improving bitumen binder performance by improving thermal sensitivity, flexibility, stability, and stripping. In terms of thermal sensitivity, while at low temperatures it helps to dissipate the developed stress, preventing asphalt concrete cracking, at high temperatures it acts as a membrane, controls the asphalt flow, enhancing shear resistance [80]. In addition, its inherent elastomer properties proved that natural rubber latex has great potential for improving the long-term pavement performance of asphalt concrete [81].  
It has been found that rubberizing asphalt concrete not only increases the rut resistance, but also enhances fatigue life with the increase in resilient modulus [82; 83]. In addition to improving the mechanical and performance properties of asphalt concrete, in a study using both the Marshall method and a gyratory compact it has been shown that natural rubber decreases the optimum binder content in asphalt concretes, increasing its density and stability [84]. From another point of view, liquid natural rubber (LNR) was found to be more efficient than latex for the aim of modifying bitumen binders. This is mainly because LNR blends easier with a binder, which results in a homogeneous binder [85]. In this form, natural rubber has more benefits compared to its conventional form, and the properties of the blend are comparable to those produced with synthetic rubbers.  
Despite the benefits regarding the use of natural rubber in bitumen binder modification, there are some drawbacks that limit its use on a large industrial scale. Natural rubber is a highly valuable biomaterial compared to other biopolymers, hence natural rubber has been commercialized into synthetic rubber. This is mainly because of the huge difference between the available produced amount and the demand. From another point of view, even if several studies showed some benefits about the bitumen binders modified with natural rubbers for paving applications, there are still some doubts regarding to asphalt concrete performance and mechanical properties throughout the performance temperature range [86; 87].