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Article

Performance Characterisation and Microscopic Characteristics of Rapeseed Heavy Oil–Diatomaceous Earth Composite-Modified Asphalt

by
Yihao Chen
,
Jin Yi
*,
Jintao Yuan
and
Ruifeng Liu
School of Civil Engineering, Central South University of Forestry & Technology, Changsha 410004, China
*
Author to whom correspondence should be addressed.
Coatings 2023, 13(12), 2042; https://doi.org/10.3390/coatings13122042
Submission received: 16 October 2023 / Revised: 27 November 2023 / Accepted: 2 December 2023 / Published: 5 December 2023
Browse Figures
Versions Notes

Abstract

:
In order to further improve the performance of bio-asphalt, because of its poor high-temperature performance, another biomass material, diatomaceous earth, was employed as a composite modifier, and a composite-modified asphalt was made to increase the high-temperature performance of bio-asphalt. The optimal preparation parameters of a rapeseed heavy oil–diatomaceous earth composite-modified asphalt were identified by employing an orthogonal test design. Based on the laboratory test, the physical properties, rheological properties, and microscopic properties of the asphalt were evaluated correspondingly by utilising matrix asphalt, rapeseed heavy oil-modified asphalt, and diatomaceous earth-modified asphalt as the control group. The results of the orthogonal test analysis showed that the optimum preparation parameters of the rapeseed heavy oil–diatomaceous earth composite-modified asphalt were 8% rapeseed heavy oil, 5% diatomaceous earth, a shear period of 35 min, and a shear rate of 2500 r/min. The addition of rapeseed heavy oil improved the fatigue resistance and low-temperature performance of the asphalt, but, at the same time, the asphalt penetration increased, the softening point and viscosity decreased, and the high-temperature rutting resistance decreased. Compared with the matrix asphalt, the viscosity of the rapeseed heavy oil–diatomaceous earth composite-modified asphalt at 135 °C rose by 23.2%. The rutting factor G*/sinδ increased by 45.5%, 15.6%, 17.6%, 29.8%, and 22.0%, while the fatigue factor G*·sinδ increased by 41.9%, 14.2%, 16.7%, 19.4%, and 23.1%, respectively, in the high-temperature rheological properties test temperature interval from 52 °C to 76 °C. The creep stiffness S fell by 16.2%, 36.1%, and 25.2%, while the creep rate m rose by 25.8%, 52.9%, and 13.4%, respectively, in the low-temperature rheological performance test temperature interval from −24 °C to −12 °C. Therefore, diatomaceous earth may effectively counteract the softening effect of the rapeseed heavy oil on the matrix asphalt and may raise the strength level and permanent deformation resistance of the composite-modified asphalt with only partial loss of fatigue resistance. The matrix asphalt, rapeseed oil, and diatomaceous earth exhibited high compatibility. The integration of rapeseed oil and diatomaceous earth largely did not modify the chemical properties of asphalt, and it was able to maintain the qualities of asphalt itself. Rapeseed heavy oil and diatomaceous earth on the thermal stability of the matrix asphalt has the opposite effect. The reorganisation component of diatomaceous earth on the colloidal structure of asphalt is conducive to the stabilisation of the nature of the asphalt, which can significantly improve the temperature stability of asphalt.

1. Introduction

The high demand for the usage of petroleum asphalt for asphalt pavement construction and maintenance and the non-renewable status quo of petroleum asphalt have greatly hindered the sustainable development of future road infrastructure [1]. In order to cope with the worldwide limitation of petroleum resources, to achieve the sustainable development of asphalt pavement technology based on the use of biomass resources, bio-asphalt material technology has garnered significant attention in recent years [2]. Bio-asphalt is a combination of biomass heavy oil from the rapid cracking of biomass such as crop straw, waste oil and grease, energy plants and livestock manure, and petroleum asphalt or external dopant, which has the significant advantages of huge reserves, environmental protection and regeneration, a wide range of sources, and low price [3]. Bio-oil and petroleum asphalt have similar chemical compositions and good compatibility, as new road engineering materials have been intensively researched in recent years for the complete or partial replacement of non-renewable petroleum asphalt [4]. Studies have shown that bio-oil can improve the low-temperature cracking resistance, water damage resistance, and fatigue resistance of asphalt pavement, and it can be used to restore the road performance of aged asphalt, which has a wide range of application prospects as a binder, modifier, warm mixer, and rejuvenation agent in road engineering [5,6].
In recent years, researchers have carried out a lot of work on the features of bio-asphalt and its application potential. For the modification effect of bio-asphalt, the high- and low-temperature performance and fatigue resistance of bio-oil-modified asphalts with varying doses have been intensively examined. Peralta J et al. [7] blended bio-oil generated from the thermal cracking of red oak with different grades of matrix asphalt, and the investigation showed that the low-temperature performance of bio-asphalt has been enhanced, but there is a noticeable weakness in the high-temperature performance. Wen et al. [8] noticed that the inclusion of the bio-binder would result in a small reduction in the asphalt’s resistance to rutting. Zeng et al. [9] discovered that castor oil increased the low-temperature cracking resistance of asphalt, but it also led to a drop in the high-temperature rutting resistance. Martin Hugener et al. [10] showed that vegetable oil-based rejuvenators are well suited for the reactivation of the old binder in the reclaimed asphalt, while the highest UCT-resistance was obtained with pure rapeseed oil. Alvaro G et al. [11] prepared bio-asphalt using rapeseed oil and fish oil as bio-oil sources and paved a test section in Iceland; the results of the study showed that the two types of bio-asphalt had better low-temperature performance than the matrix asphalt, but the bonding between the bio-asphalt and the aggregate was weakened. Two types of road asphalt fluxed with two different oxidised methyl esters of rapeseed oil were used in Jan B Król’s [12] study. The test results showed that, in the temperature range of 0~40 °C, the bio-flux resulted in a much higher increase in the phase angle than in temperatures above 40 °C in the asphalt binder. The increase in the proportion of the viscous component in the low and medium binder temperature is favourable due to the potential improvement in the fatigue resistance of the asphalt mixture with such binders. Iwański M et al. [13] showed that rapeseed oil has good compatibility with matrix asphalt and makes it more homogeneous, but it also reduces the stiffness of the asphalt. Wang et al. [14] observed that the chemical functional groups of the waste food oils and the matrix asphalt were roughly the same, and after the mixing of both, the asphalt’s fatigue life would increase, but the stiffness and rutting resistance would be greatly lowered. Gokalp et al. [15] examined the modification effect of waste vegetable oil obtained by the continuous thermal cracking process on the matrix asphalt, and the results showed that 2%~10% dosing of waste vegetable oil greatly reduced the rutting factor of matrix asphalt. The use of bio-oil for the regeneration of aged asphalt is another research hotspot at present. Zaumanis et al. [16] pointed out that bio-oil has a regeneration impact on aged asphalt, and the rutting performance and low-temperature cracking resistance of regenerated asphalt mixtures have been greatly enhanced. In 2017, Nayak et al. [17] employed a variety of vegetable oils to generate a composite regeneration agent, and they discovered that when the dose of the composite regeneration agent was 5%, the anti-fatigue and high-temperature performance may be restored to the level before ageing. In 2019, Liu et al. [18] employed lignin oil as a rejuvenating agent to restore aged asphalt, and the results revealed that bio-oil lowered the shear modulus, activation energy, and the critical cracking temperature of aged asphalt, and it resulted in a superior restoration of its colloidal structure and fluidity.
As a prospective alternative to petroleum asphalt as a sustainable asphalt pavement binding material, bio-oil has significant potential for development; however, its inadequacies of poor high-temperature performance still need further investigation [19]. In addition, the numerous mechanical features of bio-asphalt, especially the long-term durability, additionally demand further research. At present, there are no mature technical approaches to increase the high-temperature stability function of bio-asphalt materials. Researchers have tried to improve the high-temperature stability and anti-ageing capabilities of bio-asphalt by adding modifiers such as polymers, organic montmorillonite, and polyphosphoric acid into bio-asphalt for the limitations of the high-temperature performance of bio-asphalt [20]. For example, in 2022, Lv S et al. [21] doped polyphosphoric acid into bio-oil-modified asphalt, and the results showed that the composite-modified asphalt had good rheological properties under high- and low-temperature conditions, and the optimal dosages of polyphosphoric acid and bio-oil were 2% and 7.5%, respectively.
Diatomaceous earth is a biogenic siliceous sedimentary rock cemented by diatom remnants and soft mud [22], with the main component being SiO2, which is characterised by its light weight, high porosity, large specific surface area, strong adsorption, and good thermal stability [23]. Researchers generally consider that the compatibility of diatomaceous earth and asphalt is better: the two can be consistently mixed to form a stable whole and they have a good modification impact on asphalt [24]. Cong et al. [25] investigated the chemical structure of diatomaceous earth-modified asphalt, demonstrating that asphalt and diatomaceous earth do not have chemical interactions, and that diatomaceous earth has a considerable boost of asphalt high-temperature anti-deformation capabilities. Tian et al. [26] investigated the effects of long-term ageing on diatomaceous earth-modified asphalt, and the results showed that the softening point increment and viscosity ageing index of diatomaceous earth-modified asphalt decreased significantly with the increase in the dosage, and the unique honeycomb structure of SiO2 in the diatomaceous earth can effectively block oxygen. Cheng Y et al. [27] discovered that the porous structure of diatomaceous earth increases the adhesion and wettability with asphalt, which helps to securely bond the asphalt. Li L et al. [28] showed that the addition of diatomaceous earth promotes the elastic recovery of asphalt and reduces the creep flexibility of asphalt.
At present, researchers generally believe that diatomaceous earth has a good modification effect on asphalt, which can give asphalt excellent road performance. Although bio-oil can replace part of the petroleum asphalt or as a modifier for asphalt, the road performance of bio-asphalt is not clear, generally showing a lack of high-temperature performance, so more research must be carried out. In order to overcome this defect, in this paper, rapeseed oil residue and the thermal cracking treatment to obtain rapeseed heavy oil as the basic raw material were investigated, along with an examination of diatomaceous earth as a composite modifier and the preparation of rapeseed heavy oil–diatomaceous earth composite-modified asphalt. The optimum preparation parameters of rapeseed heavy oil–diatomaceous earth composite-modified asphalt were identified by employing an orthogonal test design. Based on the laboratory tests, the physical properties, rheological properties, and microscopic properties of the four types of asphalt were evaluated independently using matrix asphalt, rapeseed heavy oil-modified asphalt, and diatomaceous earth-modified asphalt as control groups. The research results will provide corresponding theoretical references for the design and application of bio-oil in asphalt pavement engineering, which is of great significance and practical engineering value for realising the reuse of waste materials in the field of agriculture and animal husbandry and protecting the ecological environment. The research flow chart is shown in Figure 1.

2. Materials and Methods

2.1. Raw Materials

The matrix asphalt is 70# road petroleum asphalt. Rapeseed heavy oil was provided by Xiangtan Zhaoshan Oleochemical Technology Co. (Xiangtan, China). The diatomaceous earth is Jilin white first grade 400 mesh diatomaceous earth given by the Lingshou County Kaiyao Mineral Products Processing Factory following impurity removal and purification treatment. The essential properties of the raw materials are listed in Table 1, Table 2 and Table 3.
For the convenience of expression, in the later text, rapeseed heavy oil-modified asphalt is abbreviated as RHOMA, diatomaceous earth-modified asphalt is abbreviated as DEMA, and rapeseed heavy oil–diatomaceous earth composite-modified asphalt is abbreviated as RDCMA.

2.2. Orthogonal Experimental Design

In order to study the ideal preparation procedure of RDCMA, an L16 (43) orthogonal table planning test was used in this paper. Orthogonal test factors and levels are presented in Table 4; the orthogonal test scheme is shown in Table 5. A total of 16 RDCMAs were made according to the orthogonal test scheme, and the asphalt samples were tested for penetration, softening point, and ductility, respectively, and the results were analysed via range analysis and variance analysis. In variance analysis, the significance levels of α = 0.05 and α = 0.01 were selected, and the table reveals that F0.05(3, 6) = 4.76 and F0.01(3, 6) = 9.78. If F < F0.05, it is viewed that this factor does not have a significant influence on the test results, and it is recorded as “0”. If F0.01 > F > F0.05, it is regarded that this factor has a substantial effect on the test results, and it is recorded as “1”. If F > F0.01, the factor is considered to have a very significant effect on the test results and it is labelled as “2”.

2.3. Test Methods

(1) Physical properties: 25 °C penetration, 5 °C ductility, softening point, and Brinell’s viscosity of asphalt were examined according to the “Highway Engineering Asphalt and Asphalt Mixture Test Procedures” (JTG E20-2011) in T0604-2011, T0605-2011, T0606-2011, and T0625-2011.
(2) High-temperature rheological properties: The complex shear modulus and phase angle of asphalt specimens were tested using an Anton Paar SamrtPave Model 102 dynamic shear rheometer. The test was conducted in strain control mode, with a scanning range of 52 °C~76 °C, incremental temperature increases of 6 °C/min, and a frequency of ω = 10 rad/s. The test was conducted in the strain control mode.
(3) Low-temperature rheological properties: Asphalt specimens were tested for low-temperature rheological properties using a CANNON Instrument Company Model TE-BBR bending beam rheometer. The beam size was 127 mm × 6.35 mm × 12.70 mm and the test temperatures were −12 °C, −18 °C, and −24 °C.
(4) A Fourier transform infrared spectroscopy (FTIR): BRUKER TENSOR 37N infrared spectrometer was used to evaluate the materials, with a wavelength range of 4000 cm−1~400 cm−1, a number of scans of 32, and a resolution of 4 cm−1.
(5) Differential scanning calorimetry (DSC): The samples were tested via TA Q20 differential scanning calorimetry, with the test temperature ranging from −50 °C to 100 °C, a heating rate of 10 °C/min, a sample mass of 10 mg, and an ambient gas of nitrogen, and the data were analysed and processed using TA Universal Analysis software.
(6) Thermogravimetric analysis (TGA): The samples were tested using a PYris6 type thermogravimetric analyser produced by the PE Company of the United States, with the test temperature ranging from 0 to 800 °C, a heating rate of 20 °C/min, a sample mass of 10 mg, and an ambient gas of nitrogen, and the data were analysed and processed using TA Universal Analysis software.

2.4. Sample Preparation

The preparation procedure of the rapeseed heavy oil–diatomaceous earth composite-modified asphalt is given in Figure 2.

3. Results

3.1. Orthogonal Test Analysis

The orthogonal test results of the RDCMAs are provided in Table 6, the results of range analysis are displayed in Table 7, and the results of variance analysis are shown in Table 8.

3.1.1. Penetration

The changing law of RDCMA penetration under different factors and levels is illustrated in Figure 3. The following results can be seen:
(1) The penetration of RDCMA reduces with the rise in DE dosage, and the maximum drop approaches 8.4%. After putting DE in the asphalt mixing slurry, with its pore structure adsorption of the asphalt slurry and its high affinity components, the surface of the DE particles form an interfacial adsorption layer, improving the adhesion [29]. Therefore, the asphalt consistency increases greatly with the rise in DE dosage, and the corresponding penetration also decreases.
(2) The penetration of RDCMA showed a lowering and then increasing tendency with the extension of the shear duration. When the shear time is within 35 min, the penetration of the composite-modified asphalt steadily decreases with the extension of the shear duration. The reason for this is that the swelling interaction between the DE and the asphalt slurry enables the DE to adsorb the lightweight components, the asphalt consistency increases, and thus the penetration reduces. The composite-modified asphalt penetration increases as the shear time increases, particularly after 35 min. The reason is that when the shear time is longer, the lightweight components in the asphalt become mastic volatilised, causing damage to the internal components, causing them to age; therefore, the penetration increases with the extension of the shear duration.
(3) The penetration of RDCMA similarly shows a tendency of reducing and then increasing with the rise in the shear rate. At first, with the acceleration of the shear rate, the DE and asphalt slurry swelling reaction speed steadily accelerated; the DE in the asphalt dispersion more uniformly so the asphalt consistency increased and the penetration decreased. When the shear rate hits 2500 r/min, the penetration of composite-modified asphalt starts to increase. The main reason is that when the shear rate is too fast, the rotor of the high-speed shear and the asphalt slurry friction occurs violently between the macromolecular chain broken into small fragments, resulting in a significant reduction in the relative molecular mass, the molecular chain length shortens, and the material temperature sensibility deteriorates [30]. In addition, the significant friction between the rotor and the asphalt slurry also generates a large amount of heat, which builds rapidly under high-temperature heating, leading to the volatilisation of some components and an increase in the penetration.
(4) The major order of elements impacting the penetration of RDCMA is A > C > B in descending order, in which the combination A4B3C3 can make the penetration of RDCMA achieve the optimal level, i.e., a DE of 5%, a shear duration of 35 min, and a shear rate of 2500 r/min.
From the variance analysis results, the F values of the DE and the shear rate are greater than the critical value of F0.05, indicating that the DE and the shear rate have a significant effect on penetration, whereas the F value of the shear duration is less than the critical value of F0.05, indicating that the shear duration does not have a significant effect on penetration. In addition, the F value of the DE is bigger than the F value of the shear rate, showing that the DE plays a major role in determining the penetration.

3.1.2. Softening Point

The changing law of the RDCMA softening point under different factors and levels is illustrated in Figure 4. The following results can be seen:
(1) The softening point of the RDCMA rose with the increase in the DE dosage, showing that the high-temperature performance of the asphalt was improved. The fundamental reason is that the DE is composed of porous structure particles, it may adsorb the asphalt mastic in small molecular weights, it has strong mobility of light components, and thus asphalt mobility is inhibited. As the DE and asphalt mastic interfacial force is reinforced with the increase in the amount of DE, the high-temperature performance is improved so that the softening point rises.
(2) With the elongation of the shear duration, the softening point of the RDCMA gradually increases, but when the shear duration is higher than 30 min, the increase is relatively minor and the minimum is only 0.04%. At first, with the growth of the shear duration, the dissolving of the DE and the asphalt slurry becomes more and more sufficient, the connection between the two is strengthened, the interfacial force grows, and the asphalt high-temperature performance improves, making the asphalt softening point increase. However, after the shear duration is more than 30 min, due to the volatilisation of specific components of the asphalt mastic, there is a certain degree of ageing, so the softening point increase is reduced.
(3) The softening point of the RDCMA increases with the shear rate and then declines. Before the shear rate does reaches 2500 r/min, the softening point with the acceleration of the shear rate and gradually increase; this is because as the DE and asphalt slurry in the full shear, the mixing of the two become more and more homogeneous, the DE-absorbed asphalt slurry more and more into light components and the modified asphalt system gradually become more stable, so the softening point began to rise. However, when the shear rate exceeds 2500 r/min, the softening point of the composite-modified asphalt begins to decline; this is because that a rotational speed that is too high leads to high-speed friction between the rotor and the asphalt mastic, generating a large amount of heat, leading to the shear temperature rising too high and too fast, which is not easy to control and thus causes the decomposition of the RHO and the asphalt. In addition to that, it also destroys the molecular chain of the asphalt mastic and thus reduces the high-temperature anti-deformation ability of the asphalt.
(4) The major order of factors impacting the softening point of RDCMA is A > C > B in descending order, in which the combination A4B4C3 may make the softening point value of RDCMA achieve the optimal level: i.e., a DE of 5%, a shear duration of 40 min, and a shear rate of 2500 r/min.
From the variance analysis results, it can be seen that the F values of the DE, the shear duration, and the shear rate are more than the crucial value of F0.05. Among these, the F values of the DE and the shear rate are more than the crucial value of F0.01, which suggests that the DE and the shear rate have a very substantial effect on the softening point of the RDCMA. Moreover, the F value of the DE is as high as 15.55, which shows that the DE plays a major role in determining the softening point.

3.1.3. Ductility

The changing law of the RDCMA ductility under different factors and levels is illustrated in Figure 5. The following results can be seen:
(1) The ductility of the RDCMA diminishes with the increasing of the DE. The main reason is that because the DE belongs to the inorganic siliceous rock, any DE added to the asphalt mortar is only equally disseminated so that transitioning from the initial homogeneous system over to the dispersion of the system results in a hardening effect; thus, it will diminish the ductility. In addition, in the ductility test for the low temperature to take the external force directly on the asphalt tensile, when the asphalt pulled to a particular fineness, the resulting asphalt specimen will be due to the DE and the stress concentration, accelerating the destruction of the asphalt.
(2) The ductility of the RDCMA diminishes with the extension of the shear duration. The reason for this is that when the dosage of the DE and the RHO is determined, with the prolongation of the shear duration, the asphalt, RHO, and DE undergo dissolution, and the DE adsorbs the lightweight components, which makes the asphalt colloidal structure develop more maturely and the flow coefficient decrease, which leads to the reduction in the ductility.
(3) As the shear rate increases, the ductility of the RDCMA continually falls. The reason is that when the shear rate is small, the RHO, DE, and matrix asphalt mixing is not uniform and the DE is not fully sheared, a large part of the matrix asphalt still exists in the form of particles and even agglomerates or bottoms out, and the external stretching force is easy to appear as force concentration phenomenon, so the ductility is lower. When the shear rate is gradually raised, the mixing of the RHO, DE, and matrix asphalt becomes more and more homogeneous, and the system of modified asphalt is more and more stable, but owing to the DE is a sort of inorganic siliceous rock, which will lead to hardening of the asphalt mixed mortar. In addition, when the shear rate is too high, on the one hand, the rotor and asphalt high-speed friction will generate heat and managing the shear temperature is not simple, which easily leads to local overheating of the asphalt mastic and part of the component volatilisation, causing the asphalt to age. On the other hand, if the shear rate is too high, it would disrupt the molecular chain of the matrix asphalt and RHO, which would cause the dissolution of the matrix asphalt and the RHO, impairing the low- temperature deformation resistance of the asphalt [31].
(4) The parameters impacting the ductility of RDCMA are A > B > C in descending order of predominance, where the combination A1B1C1 can make the softening point value of RDCMA reach the ideal level: i.e., a DE of 2%, a shear duration of 25 min, and a shear rate of 1500 r/min.
From the variance analysis results, it can be seen that for the ductility of the RDCMA, the F values of the DE, shear duration, and shear rate are all greater than the critical value of F0.05, where the F value of the DE is as high as 68.412, which is much greater than the critical value of F0.01, indicating that the DE has a very significant effect on ductility, which is dominant.
From the results of the range analysis and the variance analysis, it can be seen that the most important factor affecting the RDCMA penetration, softening point, and elongation of each preparation parameter is the DE, followed by the shear rate and the shear duration, in which the DE is the absolute factor affecting the three major indicators. The results of orthogonal tests show that the penetration of the RDCMA decreases with the increase in DE, decreases, and then increases with the increase in the shear duration and shear rate; the softening point of the RDCMA increases with the increase in the DE and shear duration, increases, and then decreases with the increase in its shear rate; and the ductility of the RDCMA decreases with the increase in the three factors. Since the 5 °C ductility of the 16 combinations of the RDCMA always matches the specification requirements, it was determined that the RDCMA was manufactured by employing 8% RHO incorporation, 5% DE incorporation, a shear duration of 35 min, and a shear rate of 2500 r/min by considering all the performance indicators.

3.2. Physical Properties

The optimum preparation parameters determined by the orthogonal tests were used to prepare the RDCMA, and matrix asphalt was used as a control group to test the penetration, softening point, ductility, and Brinell’s viscosity of the matrix asphalt, RHOMA, DEMA, and RDCMA, respectively, and the results of the tests are shown in Table 9.
(1) Compared to the matrix asphalt, the penetration of 8% RHOMA increased by 15.2%, whereas the penetration of 5% DEMA decreased by 54.8%. When the RHO and DE were applied combined as modifiers to the matrix asphalt, the penetration of the RDCMA decreased by 9.30% compared to the matrix asphalt, decreased by 21.30% compared to 8% RHOMA, and increased by 42.52% compared to 5% DEMA. This suggests that the DE can greatly harden the asphalt and boost its consistency.
(2) The softening point of 8% RHOMA was reduced by 2.4 °C compared to the matrix asphalt, whereas the softening point of 5% DEMA was enhanced by 30.08%. When the RHO and DE were added together as modifiers to the matrix asphalt, the softening point of RDCMA was increased by 19.71% compared to the matrix asphalt, increased by 25.98% compared to 8% RHOMA, and decreased by 5% DEMA by 7.97%, indicating that the DE can significantly improve the high-temperature performance of asphalt.
(3) The ductility of 8% RHOMA rose by 37.7% compared to matrix asphalt, but the ductility fell by 24.6% after adding the DE to the matrix asphalt, and it continued to decline with the increase in the DE. Through the inspection of the pulled-off specimens, it was found that the specimen fracture was tip-like, and a few DE particles could be observed distributed in the fine lines, indicating that when external force is taken to directly stretch the DEMA at lower temperatures, it is easy to make the DE particles have a tip effect leading to the concentration of stress, which leads to the asphalt being pulled off in advance. The ductility of the RDCMA was raised by 17.50% compared to that of the matrix asphalt and compared to that of the 8% RHOMA, which was reduced by 14.6% and which rose by 55.8% compared to 5% DEMA. This illustrates that although DE blending drastically affects the asphalt ductility, the ductility of RDCMA still fulfils the standard requirements and is better than that of the matrix asphalt.
(4) The greater the asphalt viscosity, the smaller the shear deformation produced, the better the elastic recovery performance, and the stronger the bonding capacity. The viscosity of asphalt reduced dramatically after the addition of the RHO, which showed that the adhesive ability of the asphalt was degraded. The viscosity of the DEMA increased by 44.4% compared with that of the matrix asphalt, and the viscosity of the RDCMA increased by 39.1% compared with that of the RHOMA, which indicated that the incorporation of DE would significantly increase the asphalt adhesive property, thus enhancing the high-temperature stability.
In summary, the addition of the RHO to the matrix asphalt increased the penetration and low-temperature ductility of the asphalt and decreased the softening point and viscosity, while the DE significantly decreased the penetration and low-temperature ductility of the asphalt and increased the softening point and viscosity, indicating that the RHO is detrimental to the high-temperature performance of asphalt, whereas the DE enhances the temperature-sensitive properties and high-temperature stability of asphalt.

3.3. High-Temperature Rheological Properties

The rheological properties of asphalt are related to temperature and time; in order to characterise the deformation resistance of asphalt at high temperature, the complex shear modulus G* and phase angle δ of asphalt at high temperature were measured via DSR, and the results of the tests are shown in Figure 6, Figure 7, Figure 8 and Figure 9.
G* and δ may evaluate the viscoelasticity of asphalt. The bigger the G*, the larger the asphalt strength, and the stronger the asphalt’s capacity to resist flow deformation when it is subjected to stress. The bigger the δ, the larger the amount of viscous component of the asphalt when it is subjected to load, the more serious its irrecoverable deformation, and the more likely it is to generate permanent deformation. As can be seen from Figure 6, with the increase in temperature, the G* of each asphalt progressively decreases and the δ gradually increases. The 8% RHOMA’s G* is smaller than that of the base asphalt at different test temperatures, whereas the δ is bigger than that of the base asphalt. This indicates that the strength of the asphalt reduces after RHO mixing, the degree of unrecoverable deformation of the asphalt under loading increases, and the high-temperature performance diminishes. In contrast, the G* of the 5% DEMA were all bigger than that of the matrix asphalt, and δ were all smaller than that of the matrix asphalt. The G* and δ of the RDCMA were both better than that of the matrix asphalt and the 8% RHOMA, indicating that the DE can effectively counteract the softening effect of the RHO on the matrix asphalt, and it can significantly improve the high-temperature performance of the asphalt and enhance the level of stiffness and the resistance to the permanent deformation of the asphalt.
Simple G* and δ cannot adequately determine the high-temperature performance of asphalt, so the rutting factor G*/sinδ is introduced. The rutting factor G*/sinδ represents the deformation recovery ability of asphalt, and the bigger the value, the smaller the cumulative irrecoverable deformation of asphalt, i.e., the better the rutting resistance. As can be seen from Figure 7, compared with the matrix asphalt, the rutting factor of the 8% RHOMA is dramatically lowered by as much as 50.3% at 52 °C, demonstrating that the RHO has a considerable negative influence on the high-temperature rutting resistance and the deformation resistance of the matrix asphalt. In contrast, the DE may greatly improve the asphalt resistance to high-temperature permanent deformation, and the rutting factor of the 5% DEMA is about 2.5 times that of the matrix asphalt at 52 °C, and it grows by nearly 53.6% even at 76 °C, which significantly strengthens the rutting resistance. Although the rutting factor of the RDCMA was reduced compared with the 5% DEMA, it was still superior than that of matrix asphalt and 8% RHOMA. The rutting factor of the RDCMA increased by 45.5%, 15.6%, 17.6%, 29.8%, and 22.0%, respectively, compared to matrix asphalt over the temperature interval of high-temperature rheological property testing from 52 °C to 76 °C, which is still better than matrix asphalt and 8% RHOMA, although lower than 5% DEMA. Therefore, DE may effectively offset the softening impact of RHO on matrix asphalt and strengthen the high-temperature rutting resistance of asphalt.
(3) Fatigue factor G*·sinδ is a significant index in evaluating the fatigue resistance of asphalt, and the smaller its value, the better the fatigue performance of surface asphalt. As can be observed from Figure 8, the fatigue factor of the four asphalts gradually decreases as the temperature increases, and the tendency is similar to that of G* and the rutting factor. The fatigue factor of 8% RHOMA is lower than that of matrix asphalt at all test temperatures, which implies that blending RHO boosts the fatigue performance of the asphalt at high temperatures despite it making the asphalt drop in terms of rutting resistance. On the contrary, the fatigue factor of 5% DEMA was usually at the greatest level, indicating that the blending of DE in the matrix asphalt might boost the rutting resistance but impair its fatigue resistance. When RHO and DE were used together as modifiers, the fatigue factor of the RDCMA was lowered to a level comparable to that of the matrix asphalt, and the fatigue resistance of the asphalt was strengthened. Therefore, RHO can effectively compensate for the decrease in the fatigue resistance of asphalt via DE.
The complex shear modulus index GTS can be a more realistic evaluation of asphalt in high temperatures of the temperature-sensitive performance: the bigger the value, the better the temperature-sensitivity, so it follows that the asphalt has the lowest sensitivity in low temperatures. As can be seen from Figure 9, the size of the GTS value of each asphalt is in the order of 5% DEMA > RDCMA > matrix asphalt > 8% RHOMA. This suggests that compared with the matrix asphalt, 5% DEMA is less susceptible to changes in temperature and it has the best temperature-sensitive characteristics, while the temperature-sensitive properties of 8% RHOMA is the worst, and it is more impacted by the changes in the external temperature. When RHO and DE were combined as modifiers, the GTS values of the RDCMA were recovered to the same level as that of the matrix asphalt, demonstrating that the DE can greatly mitigate the deleterious impacts of the RHO on the temperature-sensitive characteristics of the asphalt.

3.4. Low-Temperature Rheological Properties

Relying only on physical performance indicators cannot fully explain the effect of RHO and DE on the low-temperature characteristics of asphalt, so the BBR test can be determined by the asphalt creep stiffness S and creep rate m in order to further analyse the low-temperature performance of asphalt. The creep stiffness S is a characterisation of the low-temperature flexibility of asphalt: the smaller the value of S, the better the low-temperature flexibility of asphalt, and the less likely it is to crack. The creep rate m is a characterisation of the asphalt stress dispersion ability: the larger the value of m, the better the asphalt stress dispersion ability. The creep stiffness S and creep rate m of the asphalts specimen are shown in Figure 10.
It can be seen that when the test temperature was gradually reduced from −12 °C to −24 °C, the S-value of each asphalt showed different degrees of increase and the m-value showed different degrees of decrease. At the same test temperature, the S-value of the 8% RHOMA is smaller than that of the 5% DEMA and the matrix asphalt, and the m-value is always larger than that of matrix asphalt, and the lower the temperature, the more obvious this phenomenon is. This indicates that the incorporation of RHO can effectively improve the low-temperature flexibility of asphalt and enhance the low-temperature deformation resistance of asphalt. Compared with the other three asphalts, the S-value of the 5% DEMA is always the maximum value, which indicates that DE blending brings some negative impact on the low-temperature performance of the asphalt. The S-value of the RDCMA is at low levels at different test temperatures, which is only higher than that of the 8% RHOMA at the test temperatures of −24 °C and −12 °C and it is the lowest at the test temperature of −18 °C. The S-value of the RDCMA is also at low levels at different test temperatures. In addition, compared with the other three asphalts, the m-value of the RDCMA was always the maximum at different test temperatures, which indicates that the synergistic effect of RHO and DE can effectively improve the low-temperature performance of asphalts.

3.5. FTIR

The infrared spectra of the 70# matrix asphalt, RHO, and DE are given in Figure 11. Rapeseed oil is a vegetable oil and fat containing fatty acids as the main component, and the most characteristic group is the ester group, which contains C=O and C-O-C structures. In comparing the infrared spectra of the 70# matrix asphalt and RHO, it can be seen that both of them show obvious absorption peaks in the regions of 720~970 cm−1, 1300~1770 cm−1, and 2800~3000 cm−1, and the positions display a strong consistency. It can be seen that the makeup of the RHO and matrix asphalt are very comparable, with similar chemical compositions, thus the matrix asphalt and RHO show good compatibility. The DE infrared spectra are predominantly reflected in the distinctive peaks connected to SiO2. The strong and broad peaks near 1090 cm−1 and 791 cm−1 are the asymmetric and symmetric vibration peaks of Si-O, respectively. The peak at 477 cm−1 is predominantly generated via O-Si-O antisymmetric bending vibrations. In addition, the O-H-O bending vibrational absorption peak of structured water is near 1430 cm−1 [32].
The modifications in chemical groups before and after asphalt modification were analysed via FTIR, and the infrared spectra of each asphalt sample are presented in Figure 12. Compared with the 70# matrix asphalt and DEMA, the RHOMA revealed typical C=O and C-O-C vibrational absorption peaks in the structure of the RHO grease near 1010 cm−1, 1072 cm−1, and 1739 cm−1 [33]. This functional category is predominantly formed from fatty acids, which can reflect the most essential structural features of oils and fats [34]. After blending DE with asphalt, the DEMA did not display novel absorption peaks in other wave number ranges compared with the matrix asphalt, save for the distinctive vibrational absorption peaks of O-Si-O and Si-O of SiO2 at 477 cm−1, 791 cm−1, and 1090 cm−1. Characteristic absorption peaks of fatty acids and SiO2 occurred in the infrared spectra graphs of both RDCMAs. It can be seen that, in addition to the absorption peaks of the characteristic functional groups of the modifier, the absorption peaks of the four asphalts are basically the same in other wave number ranges, indicating that the RHO, DE, and the matrix asphalt have good compatibility, the three are mainly physically co-mingled in the mixing, and the mixing of RHO and DE basically does not change the chemical properties of asphalt, and it is able to retain the characteristics of the asphalt itself.

3.6. DSC

The relationship between asphalt aggregation state and temperature can be obtained through the DSC test, and then the asphalt properties can be analysed and the magnitude and temperature range of heat absorption in the process of the phase transition of the specimen can be obtained from the DSC curve graph. The DSC curves of each asphalt are shown in Figure 13, and it can be seen that the DSC curves of the three modified asphalts are relatively flat on the whole, especially in the normal service temperature interval of the asphalt pavement, and no new special peaks appeared compared with that of the matrix asphalt, indicating that the matrix asphalt, RHO, and DE can be uniformly blended. The DSC curves of the 70# matrix asphalt and the 8% RHOMA absorbed heat in the complete process, which means that the DSC measurements of the process of the sample absorbed energy throughout the DSC test and needed greater heat flow to elevate its temperature to the same level as the reference sample. In contrast, the DSC curves of asphalt changed significantly after DE incorporation, with a distinct exothermic peak at around −2 °C for 5% DEMA and RDCMA, indicating that the asphalt crystallised. The released heat of crystallisation meant that the heat flow needs to be reduced during the DSC measurements in order to maintain the temperature of the sample at the same level as that of the reference [35].
The heat absorption peak of the asphalt DSC curve represents the enthalpy change process of the asphalt phase transition, and its area reflects the complexity of the asphalt phase transition process. The wider the size of the heat absorption peak, the larger the quantity of heat absorbed when the asphalt phase transition happens, the greater the degree of microscopic changes in the components and the more unstable the properties of the asphalt. The heat absorption peak data of each asphalt obtained by utilising the TA Universal Analysis programme are provided in Table 10. In the normal service temperature interval of asphalt, from the value of heat absorption, the 5% DEMA has the best thermal stability and the 8% RHOMA has the poorest thermal stability. The overall heat absorption of the modified asphalt increased from 7.526 J/g of the matrix asphalt to 11.43 J/g following the addition of RHO, with an increase of 51.87%, while the total heat absorption of the 5% DEMA was only 4.554 J/g, which was 39.49% lower than that of the matrix asphalt. When the RHO and DE were utilised together as a modifier blended into the matrix asphalt, the total heat absorption of the RDCMA was between 8% RHOMA and 5% DEMA, and it was better than the matrix asphalt. This suggests that RHO and DE have opposite effects on the thermal stability of matrix asphalt, and the reformation of the asphalt pulp structure by DE is favourable to the stabilisation of the asphalt component properties, which can considerably increase the temperature stability of asphalt.

3.7. TGA

The TGA test can accurately reflect the destruction of asphalt in the high-temperature state; the TGA curve of asphalt and its differential DTG curve can be obtained from the asphalt onset of its decomposition temperature, as well as the temperature of the maximum loss of weight and the percentage of weight loss, and thus the thermal stability of the asphalt can be evaluated. The TGA and DTG curves of each asphalt are given in Figure 14, and it can be seen that there are essentially three stages of zero pyrolysis: strong pyrolysis and slow pyrolysis in the pyrolysis process of asphalt [36]. The zero-pyrolysis stage is from room temperature to 250 °C, where the rate of the temperature rise is modest, the TGA curve and the DTG curve are about horizontal, the asphalt quality change is small, and the rate of the weight loss is almost zero. Intense pyrolysis stage occurs between 250 °C and 500 °C, where the TGA curve lowers abruptly, the DTG curve displays a number of strong and narrow local heat absorption peaks and exothermic peaks, and the weight loss rate rises sharply. Among them, the 70# matrix asphalt attained the maximum value at 462.77 °C, the 8% RHOMA at 437.66 °C, the 5% DEMA at 472.81 °C, and the RDCMA at 451.29 °C, suggesting that the asphalt was particularly unstable and had a large loss of mass in this temperature zone. In the slow pyrolysis stage developed at 500 °C~800 °C, the TGA curve and DTG curve gradually levelled off and the thermo-oxidative decomposition was also progressively stabilised.
From the given thermogravimetric curves, the temperatures at different weight loss states of asphalt can be derived as indicated in Table 11. The size order of temperature at reaching the maximum weight loss rate is 5% DEMA > 70# matrix asphalt > RDCMA > 8% RHOMA, and the size order of mass residue percentage is 5% DEMA > RDCMA > 70# matrix asphalt > 8% RHOMA, which indicates that DE improves high-temperature performance and ageing resistance of asphalt, whereas RHO has a negative effect. In addition, before the weight loss of 20%, the same weight loss percentage of the temperature size ranking was 5% DEMA > 70 # matrix asphalt > RDCMA > 8% RHOMA; in the weight loss of 20%, after the same weight loss percentage of the temperature size, the ranking was 5% DEMA > RDCMA > 70 # matrix asphalt > 8% RHOMA. It can be noted that the 8% RHOMA has the fastest weight loss at high temperature and the worst stability at high temperature, while the addition of DE may substantially improve the high-temperature performance of asphalt.

4. Conclusions

In this study, the optimal preparation parameters of RDCMA were found using the orthogonal test and multivariate analysis. Based on the explanation of the basic properties of RDCMA, a comparative study of the physical properties, high-temperature rheological properties, and microscopic properties of 70# matrix asphalt, RHOMA, DEMA, and RDCMA was carried out, and the following conclusions were drawn:
(1) The result of the orthogonal test analysis showed that the optimum preparation parameters of RDCMA were 8% RHO incorporation, 5% DE incorporation, a shear duration of 35 min, and a shear rate of 2500 r/min.
(2) The integration of rapeseed heavy oil strengthened the fatigue resistance and low-temperature performance of the asphalt, but, at the same time, the asphalt penetration increased, the softening point and viscosity decreased, and the high-temperature rutting resistance decreased. Diatomaceous earth can effectively counteract the softening effect of rapeseed heavy oil on the matrix asphalt, and it can enhance the strength level and permanent deformation resistance of the composite-modified asphalt with a partial loss of fatigue resistance.
(3) The base asphalt, RHO, and DE show good compatibility, and the incorporation of RHO and DE largely does not modify the chemical properties of the asphalt and keeps the qualities of the asphalt itself.
(4) RHO and DE have opposite effects on the thermal stability of matrix asphalt, and the reformation of the asphalt colloidal structure from DE is favourable to the stabilisation of the properties of the components, which greatly increases the temperature stability of asphalt.

Author Contributions

Conceptualisation, J.Y. (Jin Yi); methodology, Y.C. and J.Y. (Jintao Yuan); validation, J.Y. (Jin Yi); formal analysis, Y.C.; investigation, R.L.; resources, J.Y. (Jin Yi); data curation, Y.C.; writing—original draft preparation, Y.C.; writing—review and editing, J.Y. (Jin Yi). All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Science and Technology Innovation Program of Hunan Province, grant number 2020RC4049; the Scientific Research Start-up Fund Project for Talents Introduced by the Central South University of Forestry Science and Technology (CSUFT), grant number 2019YJ033.

Conflicts of Interest

The brand names mentioned in this paper were for the reader’s convenience only and they do not suggest any endorsement by the authors. The authors declare no conflict of interest.

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Coatings 13 02042 g001
Figure 1. Research flow chart.
Figure 1. Research flow chart.
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Figure 2. Flow chart for the preparation of the RDCMA.
Figure 2. Flow chart for the preparation of the RDCMA.
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Figure 3. Changing law of the RDCMA penetration under different factors and levels.
Figure 3. Changing law of the RDCMA penetration under different factors and levels.
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Figure 4. Changing law of the RDCMA softening point under different factors and levels.
Figure 4. Changing law of the RDCMA softening point under different factors and levels.
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Figure 5. Changing law of the RDCMA ductility under different factors and levels.
Figure 5. Changing law of the RDCMA ductility under different factors and levels.
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Figure 6. Complex shear modulus and phase angle.
Figure 6. Complex shear modulus and phase angle.
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Figure 7. Rutting factor.
Figure 7. Rutting factor.
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Figure 8. Fatigue factor.
Figure 8. Fatigue factor.
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Figure 9. Complex shear modulus index GTS.
Figure 9. Complex shear modulus index GTS.
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Figure 10. Test results of low-temperature performance of each asphalt: (a) S-value; (b) m-value.
Figure 10. Test results of low-temperature performance of each asphalt: (a) S-value; (b) m-value.
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Figure 11. Infrared spectra of 70# matrix asphalt, RHO and DE.
Figure 11. Infrared spectra of 70# matrix asphalt, RHO and DE.
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Figure 12. Infrared spectra of asphalt samples.
Figure 12. Infrared spectra of asphalt samples.
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Figure 13. DSC curve of asphalt samples.
Figure 13. DSC curve of asphalt samples.
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Figure 14. TGA and DTG curves of asphalt samples.
Figure 14. TGA and DTG curves of asphalt samples.
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Table 1. Basic parameters of 70# matrix asphalt.
Table 1. Basic parameters of 70# matrix asphalt.
ItemsUnitsRequirementsResultsMethod
Penetration (25 °C)/(100 g, 5 s)0.1 mm60~8065.6ASTM D5
Penetration index-−1.5~1.0−0.7ASTM D5
Softening point°C≥4648.2ASTM D36
Dynamic viscosity (60 °C)Pa·s≥180228ASTM D4002
Ductility (15 °C, 5 cm/min)cm≥100140ASTM D113
Flash point°C≥260289ASTM D92
Solubility (trichloroethylene)%≥99.599.74ASTM D70
Table 2. Basic parameters of RHO.
Table 2. Basic parameters of RHO.
ItemsUnitsResults
pH-2.1
Densityg/cm30.96
Solubility (trichloroethylene)%87.4
Table 3. Basic parameters of DE.
Table 3. Basic parameters of DE.
ItemsUnitsResults
SiO2%≥93.00
Fe2O3%≤1.00
Al2O3%≤2.60
TiO2%≤0.30
MgO%≤0.10
Non-silicon material%≤2.30
Oil absorption%160
Penetration dc-7.16
Packing densityg/mL0.29
Solid densityg/mL0.52
pH-≤8.70
Table 4. Orthogonal test factors and levels.
Table 4. Orthogonal test factors and levels.
LevelDE (%)Shear Duration (min)Shear Rate (r/min)
12251500
23302000
34352500
45403000
Table 5. Orthogonal test scheme.
Table 5. Orthogonal test scheme.
No.DE (%)Shear Duration (min)Shear Rate (r/min)
12251500
22302000
32352500
42403000
53252000
63301500
73353000
83402500
94252500
104303000
114351500
124402000
135253000
145302500
155352000
165401500
Table 6. Orthogonal test results scheme.
Table 6. Orthogonal test results scheme.
No.Penetration (25 °C)Softening PointDuctility (5 °C, 5 cm/min)
155.151.514.1
251.854.113.1
348.755.512.6
448.555.911.2
550.754.412.9
652.653.112.1
746.156.710.9
844.957.310.4
949.654.99.9
1044.157.410
1149.954.610.8
1248.855.79.6
1347.656.58.8
1439.659.68.5
1542.657.57.9
1646.956.88.3
Table 7. Range analysis results of the orthogonal tests.
Table 7. Range analysis results of the orthogonal tests.
ItemsFactorsK1K2K3K4k1k2k3k4R
PenetrationA204.100194.300192.400176.70051.02548.57548.10044.1756.850
B203.000188.100187.300189.10050.75047.02546.82547.2753.925
C204.500193.900182.800186.30051.12548.47545.70046.5755.425
Softening pointA217.000221.500222.600230.40054.25055.37555.65057.6003.350
B217.300224.200224.300225.70054.32556.05056.07556.4252.100
C216.000221.700227.300226.50054.00055.42556.82556.6252.825
DuctilityA51.00046.30040.30033.50012.75011.57510.0758.3754.375
B45.70043.70042.20039.50011.42510.92510.5509.8751.550
C45.30043.50041.40040.90011.32510.87510.35010.2251.100
Table 8. Variance analysis results of the orthogonal tests.
Table 8. Variance analysis results of the orthogonal tests.
ItemsFactorsSSdfMSFFαSignificance
PenetrationA96.472332.1578.393F0.05 = 4.76
F0.01 = 9.78		1
B41.662313.8873.6250
C69.232323.0776.0231
Softening pointA23.27737.75915.5502
B10.71233.5717.1561
C20.34236.78113.5902
DuctilityA43.057314.35268.4122
B5.11731.7068.1301
C3.07731.0264.8891
Table 9. Test results of physical properties.
Table 9. Test results of physical properties.
Items70# Matrix
Asphalt		8% RHOMA5% DEMARDCMA
Penetration (25 °C, 0.01 mm)65.675.634.259.5
Softening point (°C)48.245.862.757.7
Ductility (5 °C, cm)11.415.78.613.4
Viscosity (135 °C, Pa·s)0.4210.3730.6080.519
Table 10. Peak heat absorption data for asphalt samples.
Table 10. Peak heat absorption data for asphalt samples.
Asphalt SampleHeat Absorption Peak 1Heat Absorption Peak 2Total Heat Absorption (J/g)
Heat Absorption (J/g)Peak Width (°C)Heat Absorption (J/g)Peak Width (°C)
70# matrix asphalt4.937−5.06~10.652.58913.99~45.227.526
8% RHOMA7.680−1.36~28.443.75029.45~50.1711.430
5% DEMA2.9178.73~34.711.63751.88~65.204.554
RDCMA5.1435.30~31.072.22834.20~56.237.371
Table 11. Temperature of asphalt at different weight loss states.
Table 11. Temperature of asphalt at different weight loss states.
Asphalt SampleTemperature
at Maximum Rate of Weight Loss (°C)		Residue Percentage (%)Percentage Weight Loss Corresponding to
Temperature (°C)
5%10%20%60%80%
70# matrix asphalt462.771.62341.18372.55417.58493.97606.18
8%RHOMA437.661.9332.49364.31403.89490.75583.06
5%DEMA472.8115.18343.90376.12419.22507.97717.78
RDCMA451.2912.67335.79365.71410.10495.13684.66
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Chen, Y.; Yi, J.; Yuan, J.; Liu, R. Performance Characterisation and Microscopic Characteristics of Rapeseed Heavy Oil–Diatomaceous Earth Composite-Modified Asphalt. Coatings 2023, 13, 2042. https://doi.org/10.3390/coatings13122042

AMA Style

Chen Y, Yi J, Yuan J, Liu R. Performance Characterisation and Microscopic Characteristics of Rapeseed Heavy Oil–Diatomaceous Earth Composite-Modified Asphalt. Coatings. 2023; 13(12):2042. https://doi.org/10.3390/coatings13122042

Chicago/Turabian Style

Chen, Yihao, Jin Yi, Jintao Yuan, and Ruifeng Liu. 2023. "Performance Characterisation and Microscopic Characteristics of Rapeseed Heavy Oil–Diatomaceous Earth Composite-Modified Asphalt" Coatings 13, no. 12: 2042. https://doi.org/10.3390/coatings13122042

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