Effect of non-thermal effects in microwave heating on the rheology and chemical properties of petroleum asphalt

This study systematically analyses the rheological properties of typical petroleum asphalt heated by microwaves at different temperatures through viscosity, penetration, dynamic shear, low-temperature bending, and other rheological tests. The results show that the non-thermal effect of microwaves enhances the fluidity of petroleum asphalt in the high-temperature viscous flow state, becomes softer in the medium-temperature viscoelastic state, and becomes hard and brittle in the low-temperature glassy state. The effect of microwaves also causes some specific changes in the properties of bitumen, including softening point and glass transition temperature in the opposite direction, as well as a large difference in the response of bitumen of different viscosities. The chemical changes in microwave-heated bitumen were analyzed using component tests and microscopic observation. It was found that microwave heating homogenized and dispersed the asphaltene aggregates into small particles, with the heavy components significantly reduced and the light components increased.

effect of microwaves has long attracted the attention of researchers [1][2][3][4][5].However, the non-thermal effects of microwaves are still being explored in research.
In the field of polymer organic chemistry, extensive research has shown that microwaves can assist in polymer synthesis, extraction, and curing applications [6][7][8].Studies have shown that microwave electromagnetic oscillations can break chemical bonds to convert macromolecular compounds into lower molecular compounds, which shortens the reaction time and enhances energy efficiency.In the petroleum field, i.e., Bosisio [9] used microwave heating technology to extract heavy petroleum from shale and found that microwave heating, unlike conventional heating methods, reduces the molecular weight of asphaltene.i.e., Ying [10] proved that microwave action on heavy petroleum could increase the light component content of heavy petroleum over time.Robert [11] showed that microwave action for 2 min can decrease the viscosity of crude oil by 16% not by heating the crude oil but by cracking its heavy components.i.e., Zhao [12] demonstrated that electromagnetic oscillation of microwaves changes the morphology and structure of the wax crystals in heavy petroleum or residual petroleum making the paraffin wax from large single crystal particles before treatment to a much smaller structural strength of spherical particles.Therefore, the viscosity of heavy oil or residue has greatly reduced the viscosity is greatly reduced by microwave heating.i.e., Zhang [13] showed that microwave radiation reduces the content of resin and asphaltene in the residue oil and slightly increased the aromatic hydrocarbon content.And, as Robert found, reduces the viscosity of petroleum products due to chemical depolymerization or cracking reaction.Since road asphalt, being a product of deeply processed residual petroleum has the same chemical composition and structure as residual petroleum, microwave radiation could theoretically have the same non-thermal effect on asphalt that it has on thick petroleum or residual petroleum.Indeed, the effect of ultrasound on asphalt has been confirmed in similar non-contact physical fields [14].
Usually, petroleum asphalt needs to be heated to high temperatures to facilitate construction operations on asphalt pavements.Thus, the thermal effect of microwaves has been used in the maintenance and construction of asphalt pavements.Currently, the main application of microwave heating in the field of roads is the heating of pavements after rutting repair [15][16][17] or on-site regeneration [18][19][20].The study of microwave heating to promote the self-healing of pavement cracks [21,22] is also a hot research topic in road engineering.However, the physical and chemical effects attached to the non-thermal effects of microwave heating of bitumen are not yet recognized, and such information may be significant and far-reaching for the engineering application properties of bitumen.Thus, this study analyses the changes in the rheological properties of petroleum asphalt in different phase-states (different temperature bands) and the chemical changes in asphalt after microwave heating, through component separation tests and microscopic observation, to clarify the effect of non-thermal microwave effects on the rheological properties of asphalt binders from construction to service.The mechanisms underlying the changes in the rheological properties [23,24] are also initially explained.

Materials and sample preparation for the test 2.1.1. Materials
This study first tested the basic properties of the original asphalt, according to the Chinese test method (JTG E20).The original asphalt samples used were 50#, 70#, and 90# matrix asphalt, which correspond to the three representative needle penetration classifications.The test results are shown in table 1.

Test methods
The test samples were prepared using microwave heating.The equipment was a 700-W civil microwave oven (MV) with a microwave frequency of 2540 MHz.For each preparation, 100 g of the original asphalt sample was placed in a beaker for each preparation and heated continuously in the microwave oven from room temperature to 130 ± 5 °C for as short a time as possible, not exceeding 10 min to weaken the effects of aging.The preparation conditions determined by calibration tests are listed in table 2.

Experiment protocol and methods
This study used a comparative analysis.First, the asphalt samples were prepared by microwave heating and original asphalt according to the method described in section 2.1.2.Second, the softening point test and differential scanning calorimetry (DSC) were used to measure the softening point and glass transition temperature of the different bitumen samples to determine the temperature range of the different stages of asphalt samples.Third, the rheological parameters of the bitumen in different phase states were analyzed at different temperature ranges based on the dynamic shear rheology (DSR) test (i.e., temperature scan, amplitude scan, and viscosity test), the bending beam rheology (BBR) test (i.e., test of the bending creep), and the needle penetration test.Finally, through optical microscope observation and a four-component test, the chemical changes in asphalt were analyzed from the microscopic and component perspectives.
Experimental test method: In the DSC test, a temperature rise rate of 10 °C min −1 , a nitrogen flow rate of 40 mL min −1 , a test sample of approximately 10 mg, and a temperature range of −60 °C to 80 °C were used.
The parameter values set in the DSR viscosity test: strain-controlled loading mode is selected and a 25 mm diameter rotor and base are used with a distance of 1 mm between them, the test temperature range is 30 °C to 60 °C, the shear rate is 25 m s −1 and the strain level is 1%.
The parameter values set in the temperature scanning test were: and used a 25 mm diameter rotor and base with a distance of 1 mm between them, a test temperature range of 30 °C to 60 °C, a loading frequency of 10 rad s −1 and a strain level of 1%.
The Japanese OLYMPUS CX-23 biological microscope was used to examine the microscopic state of the asphalt before and after microwave treatment.In order to obtain the best observation field and clarity, the objective lens was selected as 10X and the eyepiece was selected as 40X with a magnification of 400.
The four-component test method uses n-heptane rinsing to obtain saturated alkanes, then aromatic hydrocarbons are washed out by toluene, and finally, gums are obtained using a toluene-ethanol solution, leaving the insoluble asphaltenes adsorbed in an Al 2 O 3 column.The difference in the relative content of the components of the original and aged asphalt specimens before and after ultrasonic disposal is quantified in conjunction with the test requirements.
The test scheme, method, analysis, and evaluation process are shown in figure 1.

Change in the phase transition temperature point
The softening points of the different asphalt samples were determined directly using the ring-and-ball (R&B) method, and the glass transition temperatures were determined indirectly based on differential scanning calorimetry (DSC).The results of these tests are shown in figure 2.
The results shown in figure 2 denote that after the microwave heating, the softening points of the three asphalt samples decreased, and their glass transition temperatures increased, unlike after the original asphalt.The softening points decreased by 2 °C-3 °C on average, and the glass transition temperatures increased by about 1 °C-2 °C.Microwave heating narrowed the viscoelastic temperature range between the phase transition points.For the matrix asphalt, the aging caused by the heating process usually increases the two-phase transition points in the same direction.In this experiment, however, the changes occurred in the opposite direction, so it was judged that asphalt not only ages after microwave heating.
This modification in the phase change point due to the non-thermal effect of the microwaves predicts that the rheological properties of asphalt may change significantly at different temperature intervals depending on the softening point and the glass transition temperature.The temperature above 60 °C the viscous flow range of three kinds of asphalt, and the temperatures below −1 °C, −5 °C, and −9 °C are defined as the glass temperature range of 50 #, 70 #, and 90 # asphalt respectively.In addition, subsequent rheological analysis tests were carried out at the above temperatures.

High temperature-viscous flow state property change 3.2.1. Viscosity
The high-temperature viscosities of the three asphalt samples after microwave were determined using DSR.The results are shown in table 3.
The test results for the high-temperature viscous flow section showed that the viscosity of the microwaveheated asphalt decreased significantly.The relative reductions in the viscosities were 25%-38% at 90 °C, 13%-17% at 120 °C, and 5%-7% at 135 °C.This was evident at the lower test temperatures, and the relative change decreased when the test temperature increased.The changes in the viscosity of the 50# bitumen were more pronounced at all the test temperatures.

Temperature sensitivity
The viscosities in table 3 were measured twice logarithmically, after which the temperatures were converted to Kelvin temperatures, and the logarithms were again measured.The values were regressed to the viscositytemperature curve shown in figure 3 to analyze the changes in the viscosity-temperature relationship.
Unlike the correlation coefficient of the original asphalts, that of the microwave-heated asphalt did not change significantly after the regression to the viscosity-temperature curve, which indicates that the microwave-  heated asphalt still conformed to the properties of a high-temperature Newtonian fluid.The slope of each gradient was extracted as the high-temperature viscosity index (VTS) [25,26] and drawn on the histogram, which showed that the temperature sensitivity of the 50#, 70#, and 90# petroleum asphalts decreased.Thus, microwave heating reduced the high-temperature sensitivity of 50#,70#, and 90# asphalts.
The temperature sensitivity of matrix asphalt is closely related to its colloidal structure, which is usually the same in different temperature domains.The temperature sensitivity of the gel-biased asphalt at room temperature was lower.The relative viscosity change of microwave heated 50# asphalt in this test is relatively obvious.On the one hand, this shows that viscous asphalt may be more sensitive to the nonthermal effect of microwaves.On the other hand, this shows that such an effect must involve the chemical composition and the chemistry structure, so it must extend to other temperature domains (i.e., to other phase-states).The penetration temperatures of the three microwave heating asphalts significantly increased, with the most significant increase seen in the 50# asphalt, which is consistent with the trend of the high-temperature viscosity changes.The penetration index of the microwave-heated asphalt increased, and the temperature sensitivity of the normal-temperature viscoelastic asphalt decreased.The asphalt penetration belongs to the equivalent viscosity index at room temperature,, which reflects the soft and hard states at room temperature, and microwave heating softens the asphalt.Usually, softer asphalt favors the sol structure; its temperature sensitivity is high; and with microwave heating, its sensitivity to room temperature no longer conforms to the conventional change trend.At the same time, the change trend of 70# and 90# asphalts are consistent with high temperature sensitivity., whereas the variation trend between normal temperature sensitivity and high-temperature  sensitivity is opposite on 50 asphalt.. Similar to the findings discussed in section 3.2.2, the above phenomena indicate that the non-thermal effects of microwave heating have a higher impact on the viscoelastic properties of the viscous 50# bitumen.That difference in the molecular level of the bitumen makes it respond differently to the non-thermal effects of microwave heating, as shown by the inconsistency of the temperature sensitivity trends at high and low temperatures.

Linear viscoelastic region
The DSR was used to scan the asphalt sample for strain, with the variation of the energy storage modulus G' versus the strain plotter at the 30 °C and 45 °C test temperatures, respectively, as shown in figure 4.
Figure 4 shows that microwave heating significantly delayed the shear failure point (i.e., the bending point of the data line) to the high-strain zone and enlarged the linear viscoelastic region of the asphalt.Generally, the amplification of the linear viscoelastic region denotes the increase in the asphalt thixotropic limit, such as the difference between the linear viscoelastic region before and after the asphalt aging [27].Based on the microwave heating test, it is found that the linear viscoelastic region of the three kind of laboratory samples are delayed to the high strain region.which is beneficial for engineering applications.

Viscoelastic proportion
The usual method for analyzing the viscoelastic ratio of asphalt can be used with the complex shear modulus G * and the phase angle δ [28,29].And the test used DSR to perform temperature scanning tests on the asphalt samples with different heating methods at 30 °C.The storage modulus G' and loss modulus G' results are shown in figure 5.
The trends in the energy storage modulus G' and the loss modulus G' of the three asphalt samples in the graph are the same, with both of them decreasing in the microwave-heated asphalt, indicating a relative decrease in both the viscosity and elasticity of the asphalt.The viscosity and elasticity were analyzed by introducing Han curves [30,31] to investigate changes in the viscoelasticity ratios.The Han curves were based on a temperature scan test, where logG' and logG' were plotted as independent variables and dependent variables, respectively, and the relationships between the synergistic responses of the viscoelastic components of the asphalts were observed according to the logG'-logG' curve forms in figure 6.  Figure 6 shows that the Han curves of the microwave-heated asphalts obviously improved and came close to the diagonal (logG′′ = logG′), which indicates that microwave heating increased the ratios of the energy storage components to the loss components of the asphalts and thus, relatively enhanced the elastic properties and reduced the creep ability of the asphalts.At the same time, the slopes of the Han curves of all the microwaveheated asphalts decreased, which indicates that the relationship between the two heating methods and the temperature changes changed.
where K is the intercept of the logarithm of the needle penetration and the temperature regression line.
Figure 7 shows that the equivalent brittle point values of the three types of microwave-heated asphalts were higher than those of the by 14% (50#), 13% (70#), and 21% (90#).The trend of these changes is the same as that of the changes in the glass transition temperature.

Creep properties
In this test, the creep modulus S and the creep rate m of the three petroleum asphalts were by heating in a microwave oven and compared with original asphalts, respectively.The temperatures chosen for the BBR test were −6 °C, −9 °C, −12 °C, −15 °C, and −18 °C.The test results are shown in figure 8.
As shown in figure 8, the S values of the three types of asphalts generally increased and their m values generally decreased after microwave heating at different test temperatures in the glassy temperature section.The relative increases in the S values at different temperatures ranged from 1% to 10%, and the relative decreases in the m values ranged from 3% to 7%.Combined with the equivalent brittle point changes discussed in section 3.4.1, the increases in the S values and the decreases in the m values imply that the microwave-heated  asphalts became more elastic in the glassy state and less capable of creep and stress relaxation, thus becoming more stiff and brittle.

Analysis of microscopic and chemical changes 3.5.1. Optical microanalysis
The asphalt samples with two different heating methods were observed microscopically at 400x using an optical microscope.The results are shown in figure 9.
Observation of the same magnification micrographs revealed that clusters of large asphaltene aggregates were prevalent in the original asphalts.However, the asphaltenes in the microwave-heated samples were homogenized and dispersed into smaller agglomerates.Usually, asphaltene micelles in bitumen are bound together by hydrogen and π bonds and can form a network of blocked flow in the system.It was inferred that the non-thermal effect of the microwave high-frequency electromagnetic field vibration breaks this agglomerate state, and further chemical changes might have occurred.

Four-component test
To explore more deeply the effect of microwave heating of cracking the asphaltene colloidal particles of asphalt, among its internal chemical property changes, microwave heating component analysis and original asphalts component analysis had to be conducted.The analysis results are shown in figure 10.
In figure 10, the components of the microwave-heated asphalts show clear and regular change characteristics.The asphaltene (As) and resin (R) contents decreased by 6%-7% on average, and the aromatics (Ar) and saturates (S) increased by 6%-7% on average.The asphaltene content was reduced to about half of the original.The non-thermal effect of microwaves significantly changed the chemical compositions of the asphalts by reducing their heavy components and increasing their light components, which is called a 'cracking reaction'.From the component test, combined with the results of the microscopic observations, it was inferred that the high-frequency electromagnetic vibration of the microwaves broke the chemical bonds of the asphaltene aggregation system so that is cleaved into small molecular clusters.While the aromatic and saturate fractions within the aggregates were freed under intermolecular forces, their chemical bond-breaking effects contributed to the internal chemical reactions.
Combined with microscopic tests and macroscopic tests should be found: asphalt through the non-thermal effect in microwave heating changes the original molecular weight arrangement and reduces polydispersity, making the molecular chains of petroleum asphalt straighten and orientate under the action of low shear rates, resulting in a reduction of the elastic component, a reduction in the viscous component and an increase in mobility.And combined with the ideas of colloidal structure theory shows that: asphaltenes as the core component of the colloidal system, mainly constitute the overall elastic behavior of petroleum asphalt; colloids are also a kind of heavy component, with good plasticity and adhesion; saturated components and aromatic components, due to their low relative molecular mass and good mobility, ensure that asphalt specimens also have a certain degree of mobility at room temperature.
To further justify the macroscopic experimental phenomenon, in the viscous flow regime, the heavy component is reduced and the light component is increased due to the cleavage of asphaltene agglomerates, resulting in a smaller agglomerate distribution, which exhibits increased mobility.In the high elastic state, due to the lowering of the temperature to reduce the degree of freedom of the agglomerates, the asphalt shows more viscous properties, while the microwave heating of asphalt, breaking through the non-thermal effect of microwaves to break the asphaltene agglomerates, so that the lighter components are dispersed in the system, so the asphalt from viscous properties gradually to elastic properties.In the glassy zone, after microwave heating more number of a small volume of gum masses is free in the system, due to the low-temperature environment of its gum mass system being less free, the structural system is more stable at this time, and the asphalt becomes more brittle and harder.
These simultaneous changes in the chemical compositions and structures of the bitumens made them more fluid at high temperatures, less viscoelastic at medium temperatures, and more brittle at low temperatures.
The colloidal structures of the bitumens during melting and cooling, as well as the dissolution and recrystallization of the waxes in them, might also have been altered by the microwave interference, which, in turn, might have made the different bitumens respond differently to the nonthermal effects of the microwaves.This aspect of the regularity is still unclear and needs to be explored more deeply in the future.

Conclusion
This study showed that the non-thermal effects of the microwave heating of petroleum asphalt significantly changed the rheological properties and chemistry of the asphalt.The detailed findings are as follows.
• The non-thermal effect of microwaves lowered the softening point of the petroleum asphalt while raising its glass transition temperature, and the reverse of the two variables narrowed the temperature range of the viscoelastic state of the asphalt.
• In the high-temperature viscous flow region, the nonthermal effect of microwaves more substantially reduced the viscosity of the petroleum bitumen than of the viscous bitumen and the increased temperature sensitivity of asphalt in the high-temperature domain.
• The non-thermal effect of microwaves softened the petroleum asphalt in the medium-temperature viscoelastic state region and decreased the asphalt's medium-temperature sensitivity.The action of the microwave also enlarged the linear viscoelastic range of the petroleum asphalt and delayed the linear viscoelastic region towards the high-strain region.The storage and loss moduli of the microwave-heated bitumen decreased, but the elastic-to-viscous ratio increased.• In the low-temperature glassy range, microwave heating increased the equivalent brittle point and the bending stiffness modulus of the asphalt.The change rate of the bending stiffness modulus with the time M value decreased; that is, the non-thermal microwave effect caused the asphalt's creep ability to decline and the asphalt to become more brittle.
• The microscopic observation and the component analysis showed that microwave heating of petroleum asphalt disperses asphaltene aggregates into smaller particles and decreases the heavy components while increasing the light components.This cracking reaction caused by the non-thermal effect of microwaves significantly changes the structure of asphalt colloids, and thus, their rheological properties.

Figure 1 .
Figure 1.Flowchart of the test program methodology.

3. 3 .
Medium temperature-viscoelastic state property change 3.3.1.Penetration and temperature sensitivity The needle penetration and needle penetration index (PI) were used to characterize the viscosity and viscositytemperature relationship of asphalt in the viscoelastic state at room temperature.The needle penetration was measured at 15 °C, 25 °C, and 30 °C, and PI was calculated according to (1) below, as shown in table4.the slope of the linear regression of the logarithm of the needle penetration against the temperature.

5 .
Variation curves of the G' and G'of the (a) 50#, (b) 70#, and (c) 90# asphalts with changes in the temperature.

3. 4 .
Low-temperature-glassy state property change 3.4.1.Brittle point Based on the pin penetration given in section 3.3.1, the equivalent brittle point T1.2 values of the three bitumen types were calculated according to (2) below, and the results are shown in figure 7:

Figure 8 .
Figure 8. Modulus of stiffness S and Creep rate m valuesat different temperatures.

Figure 10 .
Figure 10.Distribution of the four components of asphalt.

Table 2 .
Heated sample preparation conditions.

Table 1 .
Basic properties of matrix asphalt.

Table 3 .
Test resultsof the high-temperature viscous flow section.

Table 4 .
Penetration and penetration index.