THE INFLUENCES OF ALTERING THE MIXING CONDITIONS ON THE PROPERTIES OF POLYMER MODIFIED BITUMEN: AN OVERVIEW

Modification of bitumen by different modifiers, causes the morphological, rheological and chemical characterisation differences related to the following three main factors: the type of the polymer, bitumen components and blending conditions. Manufacturing conditions can be investigated in several groups such as; mixing temperature, duration of mixing and shear rate (mixing speed). These manufacturing conditions are the key parameters, since they act an important role on the properties of the final product. These key parameters and their effects have always been an interesting topic for the researchers, because they may have some consequences like aging of the sample, degradation of the polymer or sometimes less absorption of the polymer by bitumen etc. This study aims to present the influences of mixing conditions on different properties of the bituminous mixtures with the help of reviewing the previous studies in the literature.


INTRODUCTION
(G*/sin(δ)), fatigue crackingparameter (G*·sin(δ)), master curves, black diagrams, Cole-Cole plots, Zero Shear Viscosity (ZSV), Multiple Stress and Recovery (MSCR) values etc., can be investigated to evaluate the rheological properties of the bitumen (Kaya et al., 2019a). Moreover, aging characteristics of the binder might be obtained by rheological investigations because, bitumen becomes to be stiffer and have different rheological properties after aging.
 Morphological Characterisations Fluorescence microscopy is used to study heterogeneous surfaces, where the components have different responses to the reflected lights. Oily fractions of the bitumen (like aromatics) have fluorescence properties, however resins and asphaltenes have no fluorescence responses under UV light (Lambert, 2005;Skoog and Leary, 1996). Neat bitumen itself has little emission, due to the fact that, all components are well mixed therefore, it cannot be observed under Optical Microscopy (OM) (Loeber et al., 1996). For this reason, it is very popular to investigate the effects of mixing conditions on the bituminous materials under OM. Fluorescence microscopy has been one of the most popular tool to investigate the morphological characterisation of PMB samples. Depending on the different coloured appearance of two phases (bitumen and polymer), it is easy to study the homogeneity and the dispersion of polymer into bitumen (Brûlé et al., 1988).
Another technique, which is common to obtain the morphological characteristics of PMB samples, is employing the Confocal Laser Scanning Microscopy (CLSM). CLSM is qualified of considering extremely localized fluorescence emissions and enables a thorough study of bitumen scale of asphalt observation (Handle et al., 2014). That capability of CLSM, allows to handle the  Thermal Characterisations Differential Scanning Calorimeter (DSC) or Modulated Differential Scanning Calorimeter (MDSC) and Thermogravimetric Analyser (TGA) are the most popular devices, which can be used for thermal analyses of PMB. DSC or MDSC is employed to characterise the physical/chemical properties, depending on the heat exchange such as Tg, heat capacity, chemical reactions or phase transition, like crystallization and melting (Soenen et al., 2014). Glass transition Temperature (Tg) is a distinctive feature of the polymeric materials. Modulated DSC (MDSC) is used in order to study the thermal behaviour of the polymer modified bitumen samples, as it is useful in the detection of overlapping Tg's (Reading et al., 1994). The distinct advantage of MDSC is that, it applies two simultaneous heating rates to the sample, a linear heating rate provides the total heat flow rate similar to standard or conventional DSC, but at the same time, a sinusoidal (modulated) heating rate provides a route to determine a fraction of the total heat flow. Bitumen has 4 different Tg corresponding the Tg's of saturates, aromatics, resins and asphaltene fractions. Therefore, PMB samples should have more than 4 Tg, depending on the polymer used for modification. Moreover, in some situations new Tg'(s) can arise/arises, from a phase of mixed compositions, which occurred by branching and cross linking between polymer and bitumen. Ageing evaluation can be done by DSC, since it allows to assess the asphaltenes fractions among the bitumen components.
Thermal gravimetric analysis is an experimental technique, which is useful for measuring the thermal stability of a material and extrapolate its maximum operating temperature above, which degradation starts to take place (Hatakeyama and Quinn, 1999). The thermal characterisation of bitumen is a very difficult investigation, because of the complex fractions of the bitumen (Yoon et al., 2009). In the literature, it can be found that, bitumen displays three major mass losses, first one because of the volatilization of light fractions (saturates and non-polar aromatics), second one depending on the decompositions of resins and asphaltenes and the last one due to the asphaltene .

THE EFFECTS OF MIXING CONDITIONS
In this section, the previous studies, dealing with the effects of mixing conditions, have been overlooked and presented depending on the different key parameters. The studies were handled as the effects of five sub grouped and they are: mixing duration, shear rate and mixing temperature.

The Effect of Mixing Duration
Haddadi et al (2008) investigated the importance of the mixing duration on the performance of bitumen, modified with different amount of EVA additive. They measured the penetration value by increasing the mixing duration up to four hours, for each sample. As a result, they deduced that, penetration value was decreased by the duration and at the end, after four hours, it became a constant value. Additionally, they did the same investigations for softening point and obtained the similar results. Therefore, they determined mixing duration as four hours for the speed of 300 rpm. Larsen et al. (2009) investigated the aging effects of the mixing conditions on the structural characteristics of the sample. They examined two different styrene-butadiene-styrene (SBS) modified samples, one mixed for 90 mins while the other for 150 mins. They compared the absorbance values, which correspond to carbonyl group and butadiene blocks, to reveal the aging effects of longer mixing. Moreover, they could be able to conclude the degradation of SBS, after mixing at higher temperature and shearing rate depending on the FTIR investigations. Fang et al. (2014) studied the effects of mixing duration of waste polyethylene modified bitumen on high temperature storage stability, thermal and morphological characteristics of the sample. TGA, DSC and fluorescence microscopy were employed for the investigations. In this study, it was found that, phase segregation changed with the mixing duration, if the mixing temperature was not high enough. Moreover, morphologies of the samples were directly affected with the key parameters. Homogeneity of the polymer phase into the bitumen phase, increased with the increasing mixing duration. However, after a specific value, PMB polymer in the PMB became to agglomerate. On the other hand, this study validated that, key parameters had a little influence on the thermal characteristics of the sample. Additionally, they suggested the optimum key parameters of the blend.
Shaffii et al. (2017), completed a study, where they reported that, the increase of the mixing duration up to 45 mins, results in the decrease of softening point and the increase of penetration value, however more than 45 mins of mixing duration results in the increase of softening point and the decrease of penetration value. Furthermore, they concluded that, the increase in the mixing duration results in the lower resistance to the temperature susceptibility. But it changes by reversal, when the sample starts to oxidise, due to the higher mixing duration.
Key parameters are not only important at mixing stage, but also crucial during sample preparation process. In the literature, many studies were conducted to exhibit the importance of the key parameters for sample preparation. For instance, Liu et al (2017) observed different morphological images under confocal laser scanning microscopy (CLSM) by altering the duration of curing for EVA modified bitumen. Initially, PMB samples have more homogenous morphologies, by increasing time, but after a specific duration, morphologies became to be phase separated. Additionally, they employed SEM for further morphological characterisation, to evaluate the effects of the curing time and the results agreed well with the results of CLSM.
Shaffie et al. (2018), studied the effects of the mixing duration on the physical properties of neat bitumen. According to the results, the higher the mixing duration, the higher the penetration value and the softening point of the sample. Moreover, they investigated the changes on the penetration indices of the sample, which is related with the temperature susceptibility. They concluded that, the increase in the mixing duration results in the higher resistance to the temperature susceptibility.  Gingras et al. (2005) examined the effects of process parameters, which are temperature, rotor speed, dispersed phase content and bitumen grade, on the droplet size in bitumen emulsion. They obtained that, average droplet size is decreased with the increment of rotor speed.

The Effect of Shear Rate
Garcia-Morales et al. (2007), investigated the possible impacts of employing the high and low shear rate mixers during manufacturing the recycled EVA/LDPE modified bitumen. According to their findings, processing with the high shear device shortened the necessary mixing time. Moreover, the PMB samples produced with high shear rates contained higher volume of polymer depending on the increased swelling. However, the results also exhibited that, the samples manufactured by high shear device had unstable characteristics during storage at high temperatures.
Larsen et al. (2009) completed a similar study by investigating the effects of manufacturing conditions on the microstructural and rheological characteristics of the SBS modified bitumen. They used different mixing temperatures and shearing rates as key parameters and employed rotational viscosity, fluorescence microscopy, size exclusion chromatography and FTIR spectroscopy. Consequently, they observed that, key parameters effected the dispersion of SBS into bitumen and, higher shearing rate and temperature causes the degradation of SBS within the PMB sample. Moreover, they concluded that, all key parameters are interdependent of each other and dependent to polymer type and molecular weight or bitumen characteristics. However, beyond 3000 rpm the behaviour of the sample changed into exfoliated structure. Moreover, height and the number of aromatic sheets in the bitumen was decreased with the accelerated shear rates, which can be associated with the oxidation. Based on the AFM results, they announced that, the amount of bee like structures were increased with the increasing shearing rates. On the other hand, crystalline index value was decreased for higher shearing rates.
Bagshaw et al. (2019) investigated the influences of different shear rates on the properties of nano-clay bitumen nanocomposites. As a result of their study, they concluded that, higher shear rates resulted in increased dispersion and exfoliation of the nanomer particles. However, rheological analyses showed that, altering the shear rates does not change the rheological properties of the sample significantly.
In the studies of Kaya et al. (2018Kaya et al. ( , 2019aKaya et al. ( , 2019bKaya et al. ( , 2020, where they evaluated the effects of high and low shear rates on the characteristics of SBS modified bitumen, the influences of SBS copolymer was more revealed by the increment of shear rate. They clarified this with the increment of absorbance of bitumen oily component by SBS with the help of higher mixing speed. The morphological images of PMB obtained by OM proved the higher SBS phase in the blend, which is produced by 3000 rpm compared to the one by 1000 rpm. Similar result were was found by the FTIR analyses. The more butadiene components revealed with the help of using 3000 rpm compared the 1000 rpm.

The Effect of Mixing Temperature
Determining the mixing temperature requires precise work, because the temperature must be high enough to melt polymer and the bitumen, but not too high to prevent the oxidation (Shenoy, 2000). Wegan (2001) investigated the high and low mixing temperature on the properties of PMB by considering the inclusion of the polymer inside the bitumen and the homogeneity of the blend. Depending on the morphological images obtained by this study, it was concluded that, low mixing temperature provides better inclusion of the polymer while, high mixing temperatures causes the inhomogeneous blend. Gingras et al. (2005) did not investigate only the effects of shear rate but also the mixing temperature. As a result, they obtained the increased droplet size with the decrement of the temperature.
Martin-Alfonso et. al (2009) studied the mixing temperature of bitumen with Polyethylene-Glycol functionalised with polymeric Methylene diphenyl diisocyanate (MDI-PEG). Atomic Force Microscopy (AFM), Thermogravimetric analyses (TGA) and thin layer chromatography (TLC-FID) were employed to highlight the effects of mixing temperature on the reaction, between the polymer and bitumen fractions. They determined that, increased mixing temperature provides 3D polymer-bitumen network and reaction ability, however, it also causes polymer degradation.
Dogan and Bayramli (2009) conducted a study about the processing temperature of the PMB samples containing three different types of polymers; LDPE, EVA and SBS. They chose 2 different mixing temperatures (150 o C and 180 o C) and compared the morphological, mechanical, rheological and thermal conductivity of the final products. Consequently, they determined that, processing temperature had a little influence on the characteristics of the samples.
Different study, which was done by Navarro et al. (2007), presented the influences of mixing temperature on the rheological characteristics of crumb tire rubber modified bitumen. As a result, they observed that higher mixing temperatures yielded an increase of dissolved/dispersed rubber.
Kok et al. (2011) studied the influences of mixing temperature along with the mixing duration and shear rates. As a result, they found that, higher mixing temperature does not have a clear effect on the softening point, viscosity and complex modulus, when the PMB samples produced by the low shear rates or mixing duration.
Shaffie et al. (2018) also investigated the mixing temperature effects on the conventional test results of the sample. The increase of the mixing temperature had the similar results with the increase of the mixing duration. Samples produced by higher mixing temperatures resulted in higher penetration values, lower softening point and temperature susceptibility up to a certain value. Above 180℃, when the bitumen starts to be oxidised, the increase of the mixing temperature created a decrease in the penetration value and increase in the softening point and the temperature susceptibility.

CONCLUSION
This study aims to present the importance of the mixing conditions and their effects on the characteristics of the PMB samples. Mixing conditions, in other words key parameters, are evaluated under three groups as mixing temperature, mixing duration and shearing rate. Depending on the literature review, the following results have been found as a result of this study; • The influences of the mixing conditions on different properties of the bituminous mixtures can be investigated by rheological, morphological, chemical and structural characterisation.
• Mixing conditions can be named as the key parameters since, they affect the characteristics of the final product. Determining the necessary and sufficient mixing conditions are not only crucial on the manufacturing stage of the PMB, but also during the sample preparation phase before doing characterisations of the sample.
• Key parameters are interdependent of each other but dependent to the type of polymer used for modification or the chemical and physical properties of the bitumen.
• Mixing duration as a key parameter may cause aging or agglomeration of the sample, if selected longer than it should be, on the other hand it is important to have a sufficient mixing duration to obtain a homogenous PMB samples.
• Using higher shear rates than needed during the PMB manufacturing may cause oxidation of the bitumen. However, decreasing the shear rate, effects the absorbance of the polymer by the bitumen. For that reason, it needs special attention to determine the shearing rate among the mixing conditions.
• Mixing temperature should be determined by considering the melting point of both the polymer and the bitumen. Also, previous studies have shown that, using high mixing temperature causes some damages on both the polymer and the bitumen because of the oxidation. Therefore, selected mixing temperature should enable the melting of the bitumen and the polymer, but avoid the aging of the sample.
As a result, mixing conditions, also known as key parameters, is crucial for the rheological, thermal, morphological or structural characteristics of the PMB. For this reason, it is important to evaluate the different combination of mixing conditions before manufacturing, in order to have PMB sample with the desired properties.