Features of structure formation in crumb rubber modified bitumen

. The research is devoted to developing the science-based technological solution for obtaining crumb rubber modified bitumen (CRMB) with improved stability. The technological solution provides a preliminary process of devulcanization of crumb rubber in hydrocarbon plasticizer in the presence of stabilizing agent and subsequent preparation of CRMB. Using fluorescent microscopy, it was established that the formation of a branched structure of crumb rubber is achieved in the waste industrial oil, indicating devulcanization processes. It was found that the most compatible with crumb rubber is a hydrocarbon plasticizer with a high content of naphthenic oil. Synthetic wax was found to be of greater interest as a stabilizing agent, and its application in an amount of 3% allows the formation of a stable CRMB structure and stabilizes the devulcanization process. The test results of obtained CRMB showed that their viscosity corresponds to the requirements for PG bitumen


Introduction
Crumb rubber obtained by shredding end-of-life tyres tires is of interest to the road construction industry from practical, environmental and economic points of view [1].
Crumb rubber contains various rubbers, the useful properties of which are used to modify bitumen binders and asphalt concrete.The introduction of crumb rubber into bitumen binder has both the obvious advantages [1,2] and negative experience of operating asphalt concrete modified with crumb rubber [3].To ensure effective interaction of crumb rubber with bitumen, several promising methods have been developed to obtain an active rubber surface with high specific surface area (strong shear effects [4], exposure to ionizing radiation [5], treatment with devulcanizer [6]).Nevertheless, for these methods there is still a problem of dispersion in bitumen medium due to a significant increase in the surface area of the rubber modifier and change in the wettability of its surface.This leads to a lack of homogeneous distribution of crumb rubber and the formation of aggregates consisting of non-wetted particles causing cracking in the low-temperature period of operation [7].In this regard, the world community is searching for other ways of modifying bitumen with crumb rubber.
During the review of scientific and technical literature, the necessity of pre-treatment of crumb rubber in plasticizers, which should contain paraffin-naphthenic substances, asphaltenes and have lower viscosity, was established [8,9].The existence of a common problem associated with ensuring effective dispersion and uniform volume distribution of modifiers both in the hydrocarbon carrier and in the modified matrix of bitumen has been established [10].
Thus, the analysis of available literature shows that currently there is no unified scientific understanding of the physical and chemical mechanisms leading to the production of stable CRMB.In most cases, the authors describe the process of thermomechanical plasticization of crumb rubber in the following way: during the joint thermomechanical treatment, the crumb rubber swells in the oil fractions of bitumen, which weakens the intermolecular bonds in the rubber.Because of continuing heat input and mechanical effects, these weakened bonds are broken, i.e. rubber devulcanization with the formation of rubber substance, which diffuses into bitumen and structures it [11,12].The schematic mechanism of thermos-mechanical plasticization of crumb rubber is presented in Figure 1.The structuring function of rubber substance explains the effects of increasing the deformation stability of rubber-bitumen binder and asphalt concrete based on it.This research is devoted to the developing of a science-based technological solution for producing CRMB with enhanced performance properties.

Materials and Characterization
Crumb rubber fraction 0.5 mm (CR 0.5) is obtained by crushing and grinding waste rubber and technical products -pneumatic tires of passenger vehicles.With the gradual removal of textile, synthetic and metal cords.Manufactured by LLC Chekhov Regeneration Plant, Chekhov, Russia.Hydrocarbon plasticizers were considered liquid hydrocarbon carriers: 1. Medium-viscous petroleum residual extract produced by LLC "LUKOIL-Volgogradneftepererabotka", Volgograd, Russia.
2. Waste industrial oil after use in the unit of the ammonia production shop.Waste industrial oil was provided by the Azot Branch of URALCHEM, JSC, in Berezniki, Perm Region, Russia.
Physicochemical properties of hydrocarbon plasticizers are presented in Table 1.The devulcanizing agents considered were: 1. Poly-transoctenamer rubber (TOR), which is produced based on cyclooctene and has a high proportion of trans-double bonds, is produced in Germany; 2. Synthetic wax obtained by Fischer-Tropsch synthesis from natural gas in special reactors produced in Russia.
For the preparation of CRMB oil road bitumen grade BND 50/70, produced by LLC "LUKOIL-Nizhegorodnefteorgsintez" (Kstovo, Russia) was used.Bitumen was tested for compliance with the requirements of Interstate Standard GOST 33133-2014.The results of laboratory tests of the physical and mechanical properties of bitumen are shown in Table 2.

Method for Determination of Group Hydrocarbon Composition of Plasticizers
Liquid adsorption chromatography with gradient displacement on the laboratory unit "Gradient M" by SUE INHP RB was used to determine the group hydrocarbon composition of plasticizers.Installation "Gradient M" is designed for the quantitative determination of the hydrocarbon composition of heavy oil fractions -oils, vacuum gas oil, fuel oils, tar, cracking residuals, oxidized and natural bitumen.The essence of the method consists of a stepwise gradient-displacement separation of high-boiling heavy oil products into seven groups, followed by their registration with a thermal conductivity detector.The recording of the detector signals on the monitor screen is a chromatogram, with each mixture group corresponding to a specific peak.Three measurement samples were taken from each plasticizer.The standard deviation for all samples was not more than 4%.

Differential Scanning Calorimetry of Crumb Rubber
The mixing temperature of the "hydrocarbon plasticizer -crumb rubber" dispersion system was determined by thermal analysis using a LINSEIS DSC PT-1600 (Linseis GmbH, Selb, Germany) -high high-temperature differential scanning calorimeter.The crumb rubber sample was subjected to differential scanning calorimetry (DSC).A thermo-analytical technique in which the difference in the amount of heat required to raise the temperature of the sample and the standard is measured as a function of temperature.The main property measured with DSC is the heat flux, the flow of energy into/from the sample as a function of temperature or time, usually shown in units of mJ/s on the y-axis.In DSC, thermal changes occurring in the rubber particles result in the absorption (endothermic process) or release (exothermic process) of heat.Endothermic changes include evaporation, phase changes such as melting, sublimation, transition between two different crystalline structures, decomposition, etc.In contrast, exothermic changes include crystallization, chemisorption, oxidation-reduction, etc.Thus, any change of state can be detected by measuring the temperature difference.Two samples were taken from crumb rubber for research.The standard deviation was not more than 5%.

Methods for the Preparation and Characterization of Dispersion Systems
The methodology for the "hydrocarbon plasticizer -crumb rubber" dispersion system is as follows.The required plasticizer and crumb rubber amount is weighed in the first step.The plasticizer is then poured into a container with a sealed lid into which a mixer and a heat control sensor are immersed.The mixer is turned on at 100-300 rpm, and the dispersion system is heated to 210 °C.Then at the second stage, when the set temperature is reached in a container with a plasticizer at a stirring speed of 300 rpm, crumb rubber is gradually introduced for 10-15 min.The container is hermetically sealed, and the devulcanization process starts, lasting no longer than 6 h.Samples are taken every hour.
The devulcanization process of crumb rubber, accompanied by an increase in its initial volume (swelling), was studied by determining the dispersion system's hydrocarbon group composition and shear viscosity during preparation.A DSR dynamic shear rheometer based on the principle of adjustable shear strain was used to determine shear viscosity to measure flow properties.The determination of shear viscosity was carried out using a geometry (two discs) where the pad diameter was 25 mm.On the dynamic rheometer, the test temperature was 135 °C, 10 rad/s.At least three samples are prepared and tested for each percentage of crumb rubber.The standard deviation was not more than 4%.
The uniform distribution of the crumb rubber in the volume of the hydrocarbon plasticizer was assessed by fluorescence microscopy on the MIKMED-2 Luminescence Microscope instrument (LLC "Leningrad Optical-Mechanical Association", Saint Petersburg, Russia).

Methods for the Preparation and Characterization of CRMB
The methodology for the preparation of CRMB is as follows.In the first stage, the necessary amount of the "hydrocarbon plasticizer -crumb rubber" dispersion system and bitumen base are weighed.The components are then poured into a container with a sealed lid, where an anchor-type mixer and a heat control sensor are immersed.The mixer is started at 100-300 rpm, and the system is preheated to 190 °C.Once 190 °C is reached, the system components are stirred at a mixing speed of 300 rpm for one hour.In the second step, the system's temperature is reduced to 175 °C and the devulcanizing agent is gradually introduced at a stirring speed of 300 rpm over 2-3 min.The container is then closed, and the system with the devulcanizing agent is stirred for 15 min at 175 °C and a stirring speed of 300 rpm.At the end of the preparation of the rubber asphalt binder, the container is removed from the heating plate, and the system is cooled down to room temperature while periodically stirring the CRMB with a glass rod.The methodology for the preparation of CRMB is shown schematically in Figure 2.After the CRMB samples were obtained, the dynamic viscosity was tested with a rotary viscometer.If the dynamic viscosity was less than 3 Pa*s at a test temperature of 135 °C, the basic dependencies of the properties were established.If the viscosity was more than 3 Pa*s, the sample was not used in further tests.
Determination of the influence of formulation and technological factors of rubber modifier on structure parameters and properties of the modified bitumen binder -CRMB (at least three samples are prepared and tested for each percentage of crumb rubber and devulcanizing agent.Standard deviation was not more than 3%), will be carried out by methods specified in the regulatory documents governing the quality of Russian Interstate Standard GOST R 58400.1-2019(in accordance with AASHTO M 320): 1. to establish the upper limit of the operating temperature range of CRMB (PG X grade) will set the maximum temperature at which CRMB can retain the necessary properties according to the methodology set out in Russian Interstate Standard GOST R 58400.3-2019(in accordance with AASHTO R 29); 2. the resistance to plastic deformation of CRMB, which contributes to resistance to plastic rutting (in summer and spring-summer periods), will be established by determining the shear stability G*/sin δ -an index of bitumen binder's ability to resist shear effects, determined by the ratio of the complex shear modulus G* to the sine of the phase angle δ.Tests of Original Binder and Rolling Thin-Film Oven Residue will be carried out according to the methodology set out in Russian Interstate Standard GOST R 58400.10-2019(in accordance with AASHTO T 315); 3. the uniformity of the crumb rubber distribution in the volume of the CRMB will be assessed by fluorescence microscopy.Fluorescence microscopy is performed using a MIKMED-2 Luminescence Microscope equipped with a high-pressure mercury ultraviolet lamp.This method is a simple analytical technique for evaluating the morphological characteristics of crumb rubber-modified systems.A small amount of heated sample was loaded and thoroughly crushed between two slides.The slide with the sample was then cooled to room temperature and viewed under a microscope with a magnification of 500× in the MIKMED-2 Luminescence Microscope program.
At the same time, it is hoped that the research on this subject can provide additional reference value to the productivity of CRMB.

Results and Discussion
From the analysis of the chromatograms and the peaks obtained, the group compositions of the hydrocarbon plasticizers were determined, Table 3. Thermal analysis for the crumb rubber was carried out to determine the temperature of the "hydrocarbon plasticizer -crumb rubber" dispersion system.Differential Scanning Calorimetry (DSC) results are shown in Figure 3.According to the DSC curve for the studied crumb rubber, it can be concluded that the thermo-oxidation of the studied crumb rubber occurs in four stages: (1) endothermic stage (with heat absorption) at 24-190 °C, associated with evaporation of air moisture and other low molecular weight products; (2) exothermic stage (with heat release) at 190-425 °C, identified with the main period of thermal oxidation of the material, proceeding up to 378 °C, and the related evaporation of the formed oxidation products, indicating the beginning of the destructive process of rubber granules; (3) endothermic stage at 425-470 °C with peak at 442 °C characterizes degradation and destruction of rubber granule; (4) exothermic stage, above 470 °C, characterized by thermal oxidation of crumb degradation products and phase changes of carbon black.
From the results (Figure 3), we can conclude that the degradation of crumb rubber is initiated at 190 °C and proceeds up to 442 °C.However, proceeding from the technical safety of the experiment, it is necessary to consider the flash point of hydrocarbon plasticizers, which for the petroleum residual extract is equal to 284 °C, and for the waste industrial oil -220 °C.Therefore, considering data on DSC for crumb rubber and flash point for plasticizers, we chose the optimum temperature of preparation -210 °C.
For a choice of the regime of preparation "hydrocarbon plasticizer -crumb rubber" dispersion system and studying the devulcanization process, we have prepared compositions (Table 4) and studied their rheological properties by determining the shear viscosity on a dynamic shear rheometer, Figure 4.The optimal preparation time for all investigated dispersion systems No. 1-4 with different percentages of crumb rubber is when the shear viscosity of dispersion systems reaches approximately the same values.The viscosity curves go to a "plateau", indicating a maximum degree of swelling of crumb rubber and the formation of structured bonds in the dispersion system [13,14].The results of shear viscosity measurements (Figure 4) show that as a result of thermo-mechanical action, reaching a "plateau" for all the systems studied occurs after three hours.Further thermomechanical action is not effective.An exception is a sample with a residual extract containing 30% crumb rubber.This is apparently due to the initially lower naphthenic oil content of 22% in the petroleum residual extract compared to 62% in the waste industrial oil.After all, naphthenic oils are the best softeners for rubber, providing a stronger swelling of rubber, their uniform distribution and playing an essential role in improving some of the structural properties of rubber.The high crumb rubber content and low naphthenic oil content led to the delamination of the system after 4 h of thermo-mechanical processing.This is indicated by the shear viscosity values (Figure 4, 4-6 h).
For practical applications, the most promising is maximally filled with crumb rubber concentrates of "hydrocarbon plasticizer -crumb rubber" dispersion systems.Therefore, the distribution uniformity was evaluated in systems with 30% crumb rubber.The analysis of the results obtained, shows that in the sample prepared with waste industrial oil after three hours of thermo-mechanical exposure, a "loosened" structure of the crumb rubber is observed, which indicates the devulcanization process taking place.It consists of partial destruction of the rubber, resulting in local destruction of its spatial structure.That is, the density of the spatial network decreases due to the disintegration of part of the transverse bonds and some of the main molecular chains [13,15].Further, to stabilize and inhibit the devulcanization process occurring at the stage of CRMB at the final stage of preparation, a devulcanizer/stabilizer is introduced.In the sample prepared with the petroleum residual extract, this effect was not observed.This is also reflected in the lower values obtained for shear viscosity (Figure 4).Therefore, it was not considered further in the study of the dependencies of the CRMB properties.
To establish the features of the influence of crumb rubber on the structure of the CRMB, the compositions were prepared (Table 5), and their performance properties were studied (Figure 5 and Table 6).Figure 5 shows a heterogeneous system where the polymer-rich phase is associated with swelling or partial degradation of crumb rubber particles in light fractions, such as saturated and aromatic, dispersed in the bitumen matrix as a spherical structure.The size of the polymer phase appears to decrease with optimal curing time and optimal content of crumb rubber.The crumb rubber copolymers are evenly distributed in the bitumen matrix [13].Analysis of fluorescence microscopy data (Figure 5) reveals that the most uniform distribution of crumb rubber and absence of aggregated particles is observed in CRMB samples with 10% crumb rubber and 3% devulcanizing/stabilising agent (regardless of its variation).It is also worth noting that CRMB samples with 15% and 20% crumb rubber content show visible system delamination, confirmed by structural homogeneity studies, so these samples were not considered further in the study.According to the test results, CRMB No. 1 and No. 4 meet the viscosity requirements for PG bitumen, their viscosity being less than 3 Pa*s.The high-temperature value of Performance Grade for CRMB No.1 prepared with poly-transoctenamer rubber is higher than that of CRMB No.4.However, it is worth noting that for CRMB No. 4 prepared with 3% synthetic wax, the Dynamic shear for Original Binder and Rolling Thin-Film Oven Residue corresponds to the same high-temperature value of Performance Grade.This demonstrates a stable structure and stabilization of the devulcanization process, which makes it promising to investigate further the effect of this synthetic wax on CRMB properties.
It is worth mentioning that scientific research is currently ongoing to adjust and optimise compositions of CRMB, select a harder bitumen base and develop technological bases to produce more concentrated rubber-containing dispersion systems to increase the operating temperature interval of the CRMB produced.

Conclusions
The hydrocarbon plasticizer with high naphthenic oil content was found to be the most compatible with crumb rubber.To study the possibility of providing stabilization and inhibition of the ongoing devulcanization process of crumb rubber, six samples of CRMB with devulcanizing/stabilizing agents were prepared and examined.Based on the results of fluorescent microscopy, it was found that the most uniform distribution of crumb rubber and lack of aggregated particles is observed in the samples of CRMB with 10% of crumb rubber and 3% devulcanizing/stabilizing agent, regardless of its variety.The CRMB samples with a higher crumb content of 15% and 20% and with synthetic wax have been found to exhibit visible system delamination.
The test results of the CRMB obtained have shown that the viscosity of the systems meets the requirements of the standard for PG bitumen.Synthetic wax as stabilising agent for crumb rubber in the binder volume is more promising than poly-transoctenamer rubber.

Fig. 2 .
Fig. 2. Schematic illustration of the methodology for the preparation of CRMB.

Table 1 .
Physical and chemical properties of hydrocarbon plasticizers.

Table 2 .
Physical and chemical properties of waste industrial oil.

Table 3 .
Hydrocarbon group composition of the plasticizers tested.

Table 5 .
Compositions of crumb rubber modified binders.

Table 6 .
Operational properties of crumb rubber modified bitumen.