JP7526806B2 - Method for producing asphalt composition and method for producing asphalt mixture - Google Patents

Description

The present disclosure relates to a method for producing an asphalt composition and a method for producing an asphalt mixture.

New methods of using lignin extracted from wood are being investigated. Paragraph 0032 of Patent Document 1 describes that lignin can be added to asphalt.

JP 2004-143243 A

As a result of studies conducted by the present inventors, the details of which will be described with reference to examples, it has been found that simply adding lignin to asphalt may result in a decrease in the stability of the resulting asphalt composition and asphalt mixture.
The problem to be solved by the present disclosure is to provide a method for producing an asphalt composition and a method for producing an asphalt mixture that can improve stability.

The present disclosure provides a method for producing an asphalt composition and a method for producing an asphalt mixture that can improve stability.

1 is a flowchart showing a method for producing an asphalt composition and a method for producing an asphalt mixture. This is a graph in which the results of the Marshall stability test of the reference example are further added to those of FIG. This is a graph in which the stability of a reference example is further added to that of FIG. 1 is a graph showing the difference in durability of an asphalt mixture depending on the content of kraft lignin.

Below, a form for implementing the present disclosure (referred to as an embodiment) will be described with reference to the drawings. In the following description of one embodiment, other embodiments applicable to the one embodiment will also be described as appropriate. The present disclosure is not limited to the one embodiment below, and different embodiments can be combined with each other or modified as desired without significantly impairing the effects of the present disclosure. In addition, the same symbols will be used for the same components, and duplicate descriptions will be omitted. Furthermore, components having the same functions will be given the same names. The contents shown are merely schematic, and for the sake of illustration, changes may be made from the actual configuration within the scope of not significantly impairing the effects of the present disclosure, and some components may be omitted or modified between drawings.

Figure 1 is a flowchart showing a method for producing an asphalt composition and a method for producing an asphalt mixture. The asphalt mixture can be produced using an asphalt composition, and the asphalt composition includes lignin and asphalt. The method for producing the asphalt composition includes a first mixing step S1 and a holding step S2.

The asphalt mixture contains lignin, asphalt, and aggregate, and further contains a filler as appropriate. The method for producing the asphalt mixture includes a raw material mixing step S4 in which lignin, asphalt, aggregate, and appropriate filler are mixed at a temperature equal to or lower than a predetermined temperature that is the glass transition temperature of lignin. The raw material mixing step S4 includes, for example, a second mixing step S3 in which at least the asphalt composition produced by the first mixing step S1 and aggregate are mixed (and a filler may be further mixed as appropriate). However, the first mixing step S1 and the second mixing step S3 may be performed simultaneously, and specifically, in the raw material mixing step S4, lignin, asphalt, aggregate, and appropriate filler may be mixed simultaneously in the same system.

The first mixing step S1 is a step of mixing lignin and asphalt. The lignin is mixed in a state where it is preheated to below its own glass transition temperature (hereinafter, simply referred to as the predetermined temperature). The asphalt is also mixed in a state where it is preheated to below the predetermined temperature.

The glass transition temperature of lignin is thought to differ depending on the type of wood from which the lignin is extracted (for example, between broadleaf and coniferous trees, and even within the same coniferous tree, such as pine and cedar). There are no particular limitations on the glass transition temperature of usable lignin, but it is, for example, 100°C or higher, preferably 110°C or higher, more preferably 120°C or higher, and particularly preferably 140°C or higher, and the upper limit is, for example, 200°C or lower, preferably 190°C or lower, more preferably 180°C or lower, and particularly preferably 165°C or lower.

The glass transition temperature of lignin can be measured, for example, by thermomechanical analysis (TMA). Thermomechanical analysis detects the change in displacement or pressure due to temperature change, and can be performed using a thermomechanical analyzer (for example, Rigaku's TMA8310L). The analysis can be performed, for example, using the above-mentioned device with 3-5 mg of lignin powder, 150 mL/min of nitrogen gas, 49 mN load, initial temperature of 50°C, final temperature of 250°C, and heating rate of 2°C/min. During heating, the temperature at which the expansion rate changes suddenly can be taken as the glass transition temperature.

By mixing lignin and asphalt at or below the above-mentioned specified temperatures, the stability of the asphalt composition and asphalt mixture can be improved.

The mixing ratio of lignin and asphalt is not particularly limited. However, in the first mixing step S1, it is preferable to mix lignin and asphalt so that the lignin is more than 0 parts by mass and 50 parts by mass or less in a total of 100 parts by mass of lignin and asphalt. By setting the mixing ratio in this range, the function of asphalt can be easily exhibited.
In particular, the upper limit of the mixing ratio of lignin is more preferably 30 parts by mass or less. By setting the mixing ratio to 30 parts by mass or less, it is possible to particularly improve the porosity and degree of saturation in the asphalt mixture.
Furthermore, the upper limit of the mixing ratio of lignin is even more preferably 10 parts by mass or less. By setting the mixing ratio to 10 parts by mass or less, the lignin and asphalt can be sufficiently coated on the aggregate during the production of the asphalt mixture, as described in detail below.
The lower limit is, for example, 5 parts by mass or more.

The specific method of mixing lignin and asphalt is not particularly limited as long as lignin at or below the above-mentioned predetermined temperature and asphalt at or below the above-mentioned predetermined temperature can be mixed. For example, asphalt can be added while lignin is stirred in a mixing device (dry mixing), or lignin can be added while asphalt is stirred in a mixing device (wet mixing). Lignin and asphalt may also be added simultaneously to the mixing device. Asphalt and lignin circulate within the mixing device by sufficient stirring.

In particular, in the first mixing step S1, it is preferable to mix lignin at or below the specified temperature while stirring the asphalt at or below the specified temperature with a mixing device. In this way, it is possible to suppress the aggregation of lignin and to mix the lignin evenly throughout the asphalt. There are no particular limitations on the mixing device, and any mixing device can be used, for example, an asphalt mixer.

It is preferable that the temperature of the asphalt is 60°C or higher. By keeping the temperature of the asphalt at 60°C or higher, the solidification of the asphalt can be suppressed and the lignin can be dispersed in the asphalt. Therefore, it is preferable that the asphalt is mixed with the lignin at a temperature lower than the above-mentioned predetermined temperature and preheated to, for example, 60°C or higher.

The temperature of the lignin is not particularly limited and may be lower than the temperature of the asphalt being mixed, or higher than the temperature of the asphalt being mixed.

It is also preferable to use the mixing device in a preheated state, below the above-mentioned predetermined temperature and, for example, above the temperature of the asphalt to be mixed. This makes it possible to prevent the lignin and asphalt inside the mixing device from exceeding the predetermined temperature, and also to prevent the asphalt from solidifying due to a drop in temperature.

It is preferable that the first mixing step S1 is performed in an open system. Since both the lignin and the asphalt are mixed at or below the above-mentioned predetermined temperature, the decomposition of the lignin can be suppressed, and the generation of hydrogen sulfide during mixing can be suppressed. Therefore, the first mixing step S1 can be performed in an open system, and a mixing device having a simple configuration can be used.

The specific types of lignin and asphalt are not particularly limited. However, it is preferable that the lignin contains kraft lignin, and more preferably, it contains dried kraft lignin. Since dried kraft lignin is easily available, the asphalt composition and asphalt mixture can be easily produced. In addition, it is preferable that the lignin is mixed in a powder state. By mixing in a powder state, it is possible to easily blend the lignin and asphalt. However, the lignin is not limited to dry lignin, and may be hydrous lignin (e.g., wet lignin, slurry, etc.), which is lignin that contains moisture. By using hydrous lignin, the lignin can be dried during mixing inside the mixer, as will be described in detail later, and the drying time can be omitted.

The asphalt preferably contains straight asphalt. By containing straight asphalt, the durability against traffic vehicles, for example, can be improved when paving using the asphalt mixture. However, the asphalt may be modified asphalt.

It is also preferable that the asphalt contains recycled asphalt aggregate. Recycled asphalt aggregate is, for example, asphalt that has been paved but has been deformed, such as cracked or rutted, and can be produced by peeling off and appropriately crushing such asphalt. By producing asphalt compositions and asphalt mixtures using recycled asphalt aggregate, the amount of asphalt used can be reduced.

In addition, the first mixing step S1 may use any component other than lignin and asphalt as long as it does not significantly impair the effects of the present disclosure.

The holding step S2 is a step of holding the mixture at or below the above-mentioned predetermined temperature for a predetermined time after the first mixing step S1. By including the holding step S2, the asphalt composition can be cured to obtain an asphalt composition in which the lignin and asphalt are sufficiently blended. However, the holding step S2 may be omitted.

The holding step S2 is performed by keeping the temperature below the above-mentioned predetermined temperature, and it is not necessary to keep the temperature constant as long as the temperature is below the above-mentioned predetermined temperature. The lower limit of the temperature for keeping the temperature is not particularly limited, but it can be, for example, 60°C or higher. The temperature can be kept warm using any heating device.

The time period for which the holding step S2 is performed is not particularly limited, but can be, for example, 30 minutes or more and 24 hours or less. The holding step S2 may be performed by leaving the mixture stationary or with stirring. In particular, the time period for the holding step S2 can be shortened by stirring. The stirring can be performed using any stirring device.

An asphalt composition is obtained by at least going through the above-mentioned first mixing process S1 (an example of the raw material mixing process S4), and an asphalt mixture is obtained by further carrying out the second mixing process S3 (an example of the raw material mixing process S4).

The second mixing step S3 is a step of at least mixing the asphalt composition produced through at least the first mixing step S1 with aggregate. By going through the second mixing step S3, an asphalt mixture can be produced. The aggregate is, for example, sand, stone, gravel, etc. having a diameter of 1 mm or more. There is no particular restriction on the amount of aggregate used, and for example, the aggregate can be used in a ratio of 1,000 parts by mass or more to 10,000 parts by mass or less per 100 parts by mass of the asphalt composition (the total mass of asphalt and lignin).

As described above, the asphalt composition contains lignin, and lignin usually functions as a filler (or binder (binding agent such as thermosetting resin)) in the asphalt mixture. Therefore, the asphalt mixture contains at least asphalt, for example lignin as a filler, and aggregate. In the raw material mixing step S4, the use of a commonly used filler (for example, inorganic powder) can be omitted, but as described above, a general filler may be used separately. When a filler is further mixed in the raw material mixing step S4, it is desirable to mix the filler so that the filler is, for example, 50 parts by mass or less, preferably 10 parts by mass or more, and more preferably 30 parts by mass or more, per 100 parts by mass of the mixture of lignin and filler. The mixture of asphalt and filler (filler bitumen) can fill gaps between aggregates and coat the aggregates. This can improve density and stability.

The specific type of filler used is arbitrary, but examples include, but are not limited to, powdered limestone.

The specific conditions for mixing the asphalt composition and aggregate are not particularly limited. However, in the second mixing step S3, it is preferable to mix the aggregate preheated to the above-mentioned predetermined temperature or below with the asphalt composition preheated to the above-mentioned predetermined temperature or below. In this way, it is possible to easily blend the lignin and asphalt with the aggregate. When other components (separate fillers, etc.) are used, it is preferable to mix them in the same manner.

In another embodiment, the first mixing step S1 and the second mixing step S3 are performed simultaneously as described above, so that the lignin, asphalt, aggregate, and the above-mentioned filler as appropriate are mixed simultaneously in the same system. In this case, the descriptions regarding the first mixing step S1 and the second mixing step S3 can be similarly applied to the descriptions regarding the lignin, asphalt, aggregate, and the appropriate filler (usage amount, mixing temperature, etc.). For example, in the raw material mixing step S4, the lignin and asphalt can be mixed so that the lignin is more than 0 parts by mass and not more than 50 parts by mass in a total of 100 parts by mass of the lignin and asphalt.

In yet another embodiment, in the raw material mixing step S4, hydrated lignin containing lignin and water is mixed with aggregate having a temperature equal to or higher than the above-mentioned predetermined temperature, for example, in a mixer, so that the lignin is maintained at or below the above-mentioned predetermined temperature. In this way, the water in the hydrated lignin can be evaporated by the heat of the aggregate. This allows the lignin to be dried in the mixer, reducing the drying work before mixing. It is preferable to determine the water content of the lignin, the amount of other components used, the temperature, etc., so that the temperature of the mixer (especially the lignin) is maintained at or below the glass transition temperature.

The asphalt mixture produced is used, for example, for road paving, roof waterproofing, and as a waterproofing material for dams, airport runways, etc. The asphalt mixture produced adheres easily to existing concrete, making it easy to work with. In addition, in the Marshall stability test, the ratio of the maximum load and the deformation amount to the maximum load exhibited by the specimen before it breaks is 1.7 MPa/mm or more. This improves the strength of the asphalt mixture. The Marshall stability test can be performed according to the method described in the literature: "Japan Road Association, B0001 Marshall Stability Test Method, Pavement Survey and Test Method Handbook [Volume 3] (June 2007, pp. 5-15)".

The asphalt mixture of the present disclosure also contains lignin derived from wood that has absorbed carbon dioxide in the atmosphere. Therefore, carbon derived from carbon dioxide in the atmosphere is fixed in, for example, pavement, etc., and the amount of carbon dioxide in the atmosphere can be reduced. This allows both the reduction of the amount of carbon dioxide in the atmosphere and the development of infrastructure through pavement, etc., and the implementation of Goal 13 of the Sustainable Development Goals (SDGs): [Climate change] Take urgent action to mitigate climate change and its impacts. By combining the afforestation and felling cycles, the wood used can achieve Goal 15: [Terrestrial resources] Protect, restore and promote sustainable use of terrestrial ecosystems, sustainably manage forests, combat desertification, and halt and reverse land degradation and halt biodiversity loss, as well as Goal 12: [Sustainable consumption and production] Ensure sustainable consumption and production patterns.

Example 1
An asphalt composition and an asphalt mixture were prepared according to the flow chart shown in FIG. 1, and their performance was evaluated.

Kraft lignin powder was produced by the following method. First, the kraft lignin dispersed as a polymer in the black liquor was extracted by filtration and thoroughly dried in a drying oven at 50°C to obtain a dried product of kraft lignin with a moisture content of 0. Next, the dried product was thoroughly pulverized to obtain a powder that passed through a sieve with an opening of 0.0075 mm. This powder is the dried powder of kraft lignin.

The glass transition temperature of the dried kraft lignin powder was measured using the thermomechanical analysis method described above. The glass transition temperature was found to be 161°C.

Next, asphalt and lignin were mixed by the following method (first mixing step S1 (Figure 1)). 2000 g of asphalt (petroleum asphalt for paving/straight asphalt pen 60-80; straight asphalt) preheated to 160 ° C, which is below the glass transition temperature (161 ° C) of the mixed kraft lignin, was put into a mixing device equipped with an agitator and mixed while circulating inside the mixing device. The mixing device is set to be able to mix at 160 ° C, and is a device that can mix in an open system in which the inside and outside of the mixing device are connected. Then, while stirring the asphalt, 2000 g of dry powder of kraft lignin at room temperature (20 ° C) (lignin is 50 parts by mass in a total of 100 parts by mass of lignin and asphalt. Lignin: asphalt = 50: 50) was added at 100 g per minute, and after the entire amount was added, it was circulated and mixed for 1 hour while keeping it at 160 ° C.

After 1 hour, mixing was stopped and the mixture was left to stand and cured at 160°C for 12 hours (holding step S2). After 12 hours, the mixture was removed from the mixing device to obtain an asphalt composition.

71.2 g of the asphalt composition taken out of the mixing device was preheated to 160 ° C., and the entire amount was added to 1200 g of aggregate that had been preheated to 160 ° C. and mixed for 60 seconds within 1 minute. After the entire amount was added, the mixture was mixed while circulating for 180 seconds (second mixing step S3). The mixing was performed using a device that was set to be able to mix at 160 ° C., the same as the mixing device used in the preparation of the asphalt composition, and that was capable of mixing in an open system in which the inside and outside of the mixing device were connected. After 180 seconds had elapsed, the mixing was stopped and the mixture was removed from the mixing device to obtain an asphalt mixture. A total of three asphalt mixtures were prepared by repeating the same operation (n = 3). In addition, a scaled-up asphalt mixture was also prepared using 712 g of the asphalt composition and 12,000 g of aggregate so that the asphalt composition content ratio was the same (7.6 mass%).

The above asphalt compositions and asphalt mixtures were prepared while measuring the hydrogen sulfide concentration using a hydrogen sulfide concentration measuring device (XS-2200, manufactured by Shin Cosmos Electric Co., Ltd.) installed near the mixing equipment. As a result, hydrogen sulfide was below the detection limit during preparation, and no generation of hydrogen sulfide was observed.

Example 2
An asphalt mixture was prepared in the same manner as in Example 1, except that the amount of lignin used was 7.12 g and the amount of asphalt used was 64.08 g (lignin was 10 parts by mass in a total of 100 parts by mass of lignin and asphalt. Lignin: asphalt = 10:90). A total of three asphalt mixtures were prepared by repeating the same operation (n = 3). During the preparation of the asphalt composition and the asphalt mixture, hydrogen sulfide was below the detection limit in both cases, and no generation of hydrogen sulfide was observed. The same was true when the mass of the asphalt composition and aggregate was scaled up to a total of 1 t.

Example 3
An asphalt mixture was prepared in the same manner as in Example 1, except that the amount of lignin used was 21.36 g and the amount of asphalt used was 49.84 g (lignin was 30 parts by mass in a total of 100 parts by mass of lignin and asphalt. Lignin: asphalt = 30: 70). A total of three asphalt mixtures were prepared by repeating the same operation (n = 3). During the preparation of the asphalt composition and the asphalt mixture, hydrogen sulfide was below the detection limit in both cases, and no generation of hydrogen sulfide was observed. The same was true when the total amount of the asphalt composition and aggregate was scaled up to 10 kg.

<Comparative Example 1>
An asphalt composition and an asphalt mixture were prepared in the same manner as in Example 1, except that the set temperature and preheating temperature of the mixing device were changed from 160° C. to 185° C. In Comparative Example 1, the asphalt composition and the asphalt mixture were prepared while measuring the hydrogen sulfide concentration. As a result, generation of hydrogen sulfide was observed during the preparation.

<Performance evaluation>
The Marshall stability test was carried out on the asphalt mixtures of Example 1 and Comparative Example 1 to evaluate the performance regarding stability. The Marshall stability test was carried out according to the method described in the above-mentioned literature. The results are shown in FIG. 2.

Figure 2 is a graph showing the results of the Marshall stability test for Example 1 and Comparative Example 1. The vertical axis shows the stability (kN) measured by the Marshall stability test. The higher the stability value, the more stable the asphalt mixture is. The stability of the asphalt mixture of Example 1 was 13.9 kN, and the stability of the asphalt mixture of Comparative Example 1 was 10.8 kN. Therefore, it was found that the asphalt mixture of Example 1, which was produced at a temperature below the glass transition temperature of lignin, was more stable than the asphalt mixture of Comparative Example 1, which was produced at a temperature above the glass temperature of lignin.

Figure 3 is a graph in which the results of the Marshall stability test of the Reference Example are added to Figure 2. In the Reference Example, an asphalt mixture was produced in the same manner as in Example 1, except that lignin was not used and only asphalt was used, and a Marshall stability test was performed in the same manner as in Example 1. The stability of the asphalt mixture of the Reference Example was 13.85 kN.

The asphalt mixture of Example 1, which was produced below the glass transition temperature of lignin, had the same stability as the asphalt mixture of the Reference Example, which used only asphalt without using lignin. This is thought to be because the lignin is uniformly dispersed in the asphalt mixture by stirring in a mixer below the glass transition temperature of lignin, increasing stability (maintaining the same level as straight asphalt).

However, the asphalt mixture of Comparative Example 1, which was produced at a temperature above the glass transition temperature of lignin, was about 20% less stable than the asphalt mixture of the Reference Example. This is thought to be because the lignin fed into the mixer rose in temperature above the glass transition temperature, causing changes in properties such as the generation of hydrogen sulfide. As a result, the lignin in Comparative Example 1 has different properties from the lignin in Example 1, which does not yet reach its glass transition temperature, and is therefore thought to have lower stability (less effective as a straight asphalt).

As another performance evaluation, the durability of the asphalt mixture was evaluated for Examples 1 to 3 and the Reference Example in relation to the percentage of kraft lignin contained. Dynamic stability (DS) was evaluated as an index of durability. Dynamic stability was evaluated according to the method described in "Japan Road Association, B0003▲L▼ Wheel Tracking Test Method, Pavement Survey and Test Method Handbook [Volume 3], June 2007, pp. 39-56."

Figure 4 is a graph showing the difference in durability of asphalt mixtures depending on the percentage of kraft lignin contained. The vertical axis of Figure 4 is dynamic stability, with a higher value indicating higher durability. The dynamic stability was 3653 in Example 1, 1115 in Example 2, 2640 in Example 3, and 760 in the Reference Example.

All of the asphalt mixtures in Examples 1 to 3 were produced at or below the glass transition temperature of lignin. Therefore, when produced at or below the glass transition temperature of lignin, higher durability is obtained than the asphalt mixture of the Reference Example, which does not use lignin. The reason for this, as with the discussion of Figure 3 above, is thought to be that the lignin is uniformly dispersed within the asphalt mixture by stirring in the mixer, increasing stability (maintaining the same level as straight asphalt).

In addition, it was shown that among the asphalt mixtures of Examples 1 to 3, those with a higher lignin content had higher dynamic stability. This is thought to be because the higher the lignin content, the more lignin polymer groups with small relative molecular weights that melt at relatively low temperatures are contained among the polymer groups that make up the lignin. Lignin polymer groups with small relative molecular weights act as thermosetting resins when melted. The molten scum produced by melting then has a modifying effect on the straight asphalt, which is thought to improve the dynamic stability of the asphalt mixture. This effect can be achieved by adjusting the mixing temperature to suppress the generation of hydrogen sulfide due to the decomposition of lignin.

Table 1 below illustrates the differences in behavior of lignins of different relative molecular weights contained in the lignin polymer group (so-called ordinary lignin) when the glass transition temperature measured by thermomechanical analysis is, for example, 150°C.

The glass transition temperature of lignin is assumed to differ depending on the relative molecular weight, and tends to be higher as the relative molecular weight increases and lower as the relative molecular weight decreases. For example, the glass transition temperature measured by thermomechanical analysis is an average value that includes lignins of different relative molecular weights. However, for example, by measuring the glass transition temperature by thermomechanical analysis and mixing at or below the measured glass transition temperature measured by thermomechanical analysis, it is possible to suppress thermal degradation in lignins with small, medium, or large relative molecular weights. This allows the solid lignin generated by cooling the molten material to remain in the asphalt mixture, thereby achieving the effects of the present disclosure.

As further performance evaluation, the void ratio and degree of saturation were calculated for the asphalt mixtures of Examples 1 to 3. The void ratio was calculated based on the following formula (1), and the degree of saturation was calculated based on the following formula (2). The degree of saturation is the proportion of straight asphalt in the voids in the aggregate, expressed as a percentage. The results are shown in Table 1 below.

v=Vv/V×100=(1-ρm/D)×100...Formula (1)
*v is the void ratio (%), ρm is the density of the asphalt mixture (g/cm ³ ), V is the volume of the asphalt mixture (cm ³ ), and Vv is the void volume in the asphalt mixture (cm ³ ).

s=Va/(V-Vag)×100=Va/(Va+v)×100...Formula (2)
* s is the degree of saturation (%), Va is the volume ratio of asphalt (%), Vag is the aggregate volume (cm ³ ) in the asphalt mixture, and V and v are the V and v in formula (1). Synonymous with v.

Equations (1) and (2) are those described in the literature: "Japan Road Association, B0008▲L▼Method of measuring density of asphalt mixtures, Pavement Survey and Testing Method Handbook [Volume 3], June 2007, pp. 91-105."

When using an asphalt mixture for a pavement, it is preferable that the porosity of the asphalt mixture is small in order to prevent rainwater from entering. By reducing the porosity, the coefficient of permeability can be reduced. On the other hand, as shown in the above formula (2), the degree of saturation and the porosity are inversely proportional to each other. Therefore, when the porosity is reduced, the degree of saturation increases. For this reason, as shown in Examples 2 and 3, which have a small porosity and a large degree of saturation, it was found that the amount of lignin is more than 0 parts by mass and not more than 30 parts by mass per 100 parts by mass of lignin and the asphalt. By setting it in this range, the porosity can be reduced and the degree of saturation can be increased, and an asphalt mixture suitable for pavement, for example, can be obtained. The following reasons are considered to be the reasons for this.

As explained with reference to Table 1 above, the lignin polymer group includes lignins that have a small relative molecular weight and exert a modifying effect, and other lignins that remain solid and are dispersed in asphalt. The former lignins have a small relative molecular weight and melt at the glass transition temperature in Table 3 below. On the other hand, the latter lignins have a large relative molecular weight in Table 3 below.

As mentioned above, lignin functions as a filler in the asphalt mixture, and the filler behaves as an aggregate in density calculations. However, thermally degraded lignin does not regain its lignin function even when cooled, so it does not act as a filler. For this reason, when mixed at a temperature above the glass transition temperature of lignin, the asphalt mixture contains lignin (thermally degraded) that has a lower density than the aggregate, and the density decreases, resulting in a larger calculated void ratio.

By mixing at or below the glass transition temperature of lignin and keeping the lignin content at 30 parts by mass or less, the absolute amount of lignin with a relatively small molecular weight can be reduced, thereby reducing the amount of lignin that melts and deteriorates with heat, and further reducing the amount of lignin itself present in the asphalt mixture. This reduces the amount of lignin present between the aggregates during compaction, i.e., the lignin that acts as a filler, allowing the aggregates to be brought closer together and reducing the void ratio. In addition, the aggregates can be made denser, resulting in a higher density.

As mentioned above, it is preferable to reduce the void ratio, but it is also possible to increase the void ratio. For example, in order to improve visibility in rainy weather and the skid resistance of the road surface, an asphalt mixture with high drainage properties and many voids may be produced. Such an asphalt mixture can be used, for example, for paving expressways, national roads, etc.

Example 4
An asphalt composition and an asphalt mixture containing a filler were prepared. Then, for the prepared asphalt mixture, a Marshall stability test (measurement of stability [kN]) and a wheel tracking test (measurement of dynamic stability [-]) were performed by the method described in the above <Performance evaluation>, and the void ratio and the degree of saturation were calculated.

A different hydrated kraft lignin from that in Example 1 was produced by the following method. First, the kraft lignin dispersed as a polymer in the black liquor was extracted by filtration to obtain hydrated lignin (wet lignin). A portion of the hydrated lignin was sampled and dried, and the glass transition temperature of the lignin was measured by the thermomechanical analysis method described above. As a result, the glass transition temperature was 168°C.

Next, asphalt, hydrous lignin, filler (stone powder) and aggregate were mixed simultaneously by the following method (raw material mixing step S4 (Figure 1) in which the first mixing step S1 and the second mixing step S3 are performed simultaneously). Mixing was started by placing the aggregate and filler, which had been preheated to a temperature (200°C) above the glass transition temperature of lignin, into a mixing device (the same as in Example 1) that had already contained hydrous lignin. Mixing was carried out for 100 seconds. It was confirmed that the temperature of the aggregate and filler decreased due to mixing with the hydrous lignin, and that the inside of the mixing device was maintained at or below the glass transition temperature of lignin (168°C) during mixing.

The amount of each material used was 44 kg of asphalt, 50 kg of hydrous lignin (of which 40 kg was lignin), 25 kg of filler, and 891 kg of aggregate. Therefore, in the raw material mixing step S4 (FIG. 1), the filler was mixed so that the amount of filler was 38.5 parts by mass (50 parts by mass or less) per 100 parts by mass of the mixture of lignin and filler. In addition, the amount of lignin used was 47.6 parts by mass, and the amount of aggregate used was 1060 parts by mass, per 100 parts by mass of the total of lignin and asphalt.
After mixing was completed, the mixture was immediately removed from the mixer to obtain an asphalt mixture.

The above asphalt compositions and asphalt mixtures were prepared while measuring the hydrogen sulfide concentration using a hydrogen sulfide concentration measuring device (XS-2200, manufactured by Shin Cosmos Electric Co., Ltd.) installed near the mixing equipment. As a result, hydrogen sulfide was below the detection limit during preparation, and no generation of hydrogen sulfide was observed.

The stability [kN], dynamic stability [-], void ratio and saturation degree of the prepared asphalt mixture were calculated using the method described in the above <Performance Evaluation>. The results were stability of 13.0 kN, dynamic stability of 3150, void ratio of 3.0% and saturation degree of 79.4%.

Example 5
In the second mixing step S3 (FIG. 1), the filler was mixed so that the filler was 41.7 parts by mass (50 parts by mass or less) per 100 parts by mass of the mixture of lignin and filler. An asphalt mixture was prepared in the same manner as in Example 4. The stability of the prepared asphalt mixture was 14.3 kN, the dynamic stability was 7870, the porosity was 3.7%, and the saturation degree was 75.8%.

<Comparative Example 2>
The asphalt mixture of Comparative Example 2 was prepared in the same manner as in Example 4, except that the asphalt of Example 41 was used instead of lignin (i.e., lignin was not used, and an amount of asphalt equivalent to the amount of lignin used was used). The stability of the prepared asphalt mixture was 12.8 kN, the dynamic stability was 4200, the void ratio was 3.0%, and the saturation degree was 80.0%.

Comparing the results of Examples 4 and 5 with those of Comparative Example 2, it was found that replacing part of the filler with lignin did not significantly affect the physical properties of the asphalt mixture. In particular, the dynamic stability of Example 5 was greater than that of Comparative Example 2, and performance was improved. In this way, it was confirmed that lignin behaves as a filler bitumen in the asphalt mixture and can improve performance.

S1 First mixing process S2 Holding process S3 Second mixing process S4 Raw material mixing process

Claims (15)

A method for producing an asphalt composition comprising lignin and asphalt, comprising:
A method for producing an asphalt composition, comprising: a first mixing step of mixing, in an open system, the lignin having a temperature equal to or lower than a predetermined temperature that is a glass transition temperature of the lignin and the asphalt having a temperature equal to or lower than the predetermined temperature. The method for producing an asphalt composition according to claim 1, characterized in that in the first mixing step, the lignin having a temperature equal to or lower than the predetermined temperature is mixed while stirring the asphalt having a temperature equal to or lower than the predetermined temperature. The method for producing an asphalt composition according to claim 1 or 2, characterized in that the temperature of the asphalt to be mixed is 60°C or higher. The method for producing an asphalt composition according to claim 1 or 2, characterized in that in the first mixing step, the lignin and the asphalt are mixed such that the lignin is more than 0 parts by mass and 50 parts by mass or less in a total of 100 parts by mass of the lignin and the asphalt. The method for producing an asphalt composition according to claim 1 or 2, characterized in that the lignin includes a dried product of kraft lignin. The method for producing an asphalt composition according to claim 1 or 2, characterized in that the asphalt includes straight asphalt. The method for producing an asphalt composition according to claim 1 or 2, characterized in that the asphalt contains recycled asphalt aggregate. (delete) The method for producing an asphalt composition according to claim 1 or 2, further comprising, after the first mixing step, a holding step of holding the mixture at a temperature equal to or lower than the predetermined temperature for a predetermined period of time. A method for producing an asphalt mixture containing lignin, asphalt and aggregate, comprising:
The raw material mixing step includes mixing the lignin, the asphalt, and the aggregate at a temperature equal to or lower than a predetermined temperature that is the glass transition temperature of the lignin,
The raw material mixing step includes:
A first mixing step of mixing the lignin having a predetermined temperature or lower, which is the glass transition temperature of the lignin, and the asphalt having a predetermined temperature or lower in an open system;
A method for producing an asphalt mixture, comprising: a second mixing step of mixing at least the asphalt composition produced in the first mixing step with the aggregate. (delete) The method for producing an asphalt mixture according to claim 11, characterized in that in the second mixing step, the aggregate having a temperature equal to or lower than the predetermined temperature is mixed with the asphalt composition having a temperature equal to or lower than the predetermined temperature. The method for producing an asphalt mixture according to claim 10, characterized in that in the raw material mixing step, the lignin and the asphalt are mixed so that the lignin is more than 0 parts by mass and 50 parts by mass or less in a total of 100 parts by mass of the lignin and the asphalt. The method for producing an asphalt mixture according to claim 10, characterized in that in the raw material mixing step, the hydrated lignin containing the lignin and water is mixed with the aggregate having a temperature equal to or higher than the predetermined temperature so that the lignin is maintained at a temperature equal to or lower than the predetermined temperature. In the raw material mixing step, a filler is further mixed,
The method for producing an asphalt mixture according to claim 10, characterized in that the filler is mixed so that the amount of the filler is 50 parts by mass or less per 100 parts by mass of the mixture of the lignin and the filler.

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