JPH0456981A - Fixing device - Google Patents

Abstract

PURPOSE:To prevent offset from occurring even in use for a long period by providing an offset prevention layer at the surface layer of a fixing roller, and setting the change of oil absorbing rate of the offset prevention layer at a measuring value >=-5% by an oil absorbing rate change measuring method. CONSTITUTION:The fixing roller is equipped with an elastic layer 30 consisting of HTV silicon rubber and the offset prevention layer 31 on a core bar 29, and the change of oil absorbing rate of the offset prevention layer 31 is set at the measuring value >=-5% by the oil absorbing rate change measuring method. By setting the change of oil absorbing rate of the offset prevention layer 31 in such way, the fetching performance of oil of the offset prevention layer 31 can be prevented from lowering even when heat fixing is performed for the long period. As a result, mold releasing capability from the inside of the offset prevention layer 31 can be kept in a satisfactory state. In such a way, it is possible to prevent the offset from occurring extending over the long period, and a satisfactory image can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a fixing device for fixing an unfixed image on a recording material in an image forming apparatus.

[Prior Art] In recent years, electrophotographic image forming apparatuses capable of forming color images have been used as printers, copying machines, and the like.

The image forming apparatus develops a latent image formed on a photosensitive tram, which is a latent image carrier, using color toner based on color separation signals of an original, to form a toner image, and transfers the toner image to a recording material. By doing so, a one-color image is formed. By repeating the above process one by one for each color, for example, a full-color image is formed on the recording material, and the unfixed toner image formed on the recording material is stored in the image forming apparatus. The image is fixed by the installed fixing device.

As shown in FIG. 4, the fixing device has a fixing rotary body, usually a roller-shaped fixing roller 41, and a pressure roller 42, which is a pressurizing rotary body facing the fixing roller 41. The fixing roller 41 is provided with a release agent applying means 43 for applying silicone oil as a release agent to the surface of the fixing roller 41, and a cleaning means 44 for cleaning the surface, in contact with the surface. .

The fixing roller 41 has an elastic layer 46 made of HTV silicone rubber (high temperature vulcanizable silicone rubber) formed on a core metal 45 made of aluminum or the like as a base material, and further has an elastic layer 46 made of HTV silicone rubber (high temperature vulcanizable silicone rubber) on the elastic layer 46. The anti-offset layer 47 is made of silicone rubber vulcanized at room temperature and has a thickness of 2,004 mm, for example, an outer diameter of 40 mm.
It is set so that

The pressure roller 42 has an elastic layer 49 made of HTV silicone rubber formed on a core bar 48 made of aluminum, and a fluororesin layer 50 formed on the elastic layer 49. For example, it is set to 40 mm.

A halogen heater 51 as a heat generating element is disposed inside the core metal 45 of the fixing roller 41 and the core metal 48 of the pressure roller 42, and the temperature of the pressure roller 42 is detected by a thermistor 52 that is in contact with the halogen heater 51. , controls the halogen heater 51 on and off to set the temperature of the fixing roller 41 and pressure roller 42 to, for example, 1.
It is designed to maintain a constant temperature of 70°C. Further, the fixing roller 41 and the pressure roller 42 are rotationally driven in the direction of the arrow by a drive device (not shown), and the recording material P conveyed in the direction of the arrow a by a conveying device (not shown) is rotated by both rollers. It is designed to pass through without being compressed. As a result, the unfixed toner image on the L-recording material ph is fixed onto the recording material P by the heat and pressure of both rollers.

However, some of the toner may not be fixed on the recording material and may adhere to the surface of the fixing roller, resulting in a so-called offset phenomenon. Therefore, an offset prevention layer 47 is provided on the surface layer of the fixing roller, and a release agent applying means 43 and a cleaning means 44 are provided.

First, the release agent applying means 43 sucks up the silicone oil 54 having a viscosity of Ioo C3 stored in the container 53 using the upper and lower supply rollers 55A and 55B, and applies the silicone oil 54 to the offset prevention layer 47 of the fixing roller 41. It has become. The amount of silicone oil applied to the offset prevention layer 47 is adjusted by the contact angle and pressure of the oil amount adjustment plate 56 brought into contact with the E supply roller 55A. Also,
The cleaning means 44 removes toner offset from the offset prevention layer 47 of the fixing roller 41.
It is arranged so as to be in sliding contact with the offset/sheet prevention layer 47.

With the above configuration, it has been possible to suppress the occurrence of offset at the beginning of use.

[Problems to be Solved by the Invention] However, although the above-mentioned conventional fixing device had sufficient mold release performance in the initial stage, offset occurred after about 20,000 sheets were heated.

As a result of investigating the cause of the occurrence of the offset, it was found that one cause was a decrease in the oil absorption rate of the anti-offset layer, which is the surface layer.

Here, the oil absorption rate is determined as follows. That is, dimethyl silicone oil 100
C3 (for example, KF96100 C3 manufactured by Shin-Etsu Chemical Co., Ltd.
) to 180°C, immerse a rubber piece in it, measure the weight D (g) of the rubber piece before immersion, and the weight E (g) after 8 hours of immersion. Calculated from ratio (%) = ((E-D)/D)X100. In addition, if the rubber piece already contains oil or white matter at the beginning, the oil content is extracted with a solvent such as toluene and the measurement is performed using the rubber piece.The oil absorption rate of the surface layer where offset has occurred was investigated. Initially, it was 4%, but it had dropped to 30%.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems and provide a fixing device that does not cause offset even when used for a long period of time.

[Means for Solving the Problems] According to the present invention, the above object is to provide a fixing device that is equipped with a fixing rotor having a heating element inside and fixes an unfixed developer image on a recording material. This is achieved by having an anti-offset layer on the surface layer, and the change in oil absorption of the anti-offset layer is set to less than 5% of the value measured by the oil absorption change measurement method.

[Function] According to the present invention, the offset prevention layer on the surface layer of the fixing rotor is set to 15% υ higher than its oil absorption rate change or the value measured by the oil absorption rate change measurement method, so it cannot be heated for a long period of time. Even when fixing is performed, the oil uptake ability of the anti-offset layer does not decrease. As a result, the mold releasability from inside the offset prevention layer is maintained in a good state, and no offset occurs.

[Embodiments] First to fifth embodiments of the present invention will be described with reference to FIGS. 1 to 3 of the accompanying drawings. Note that parts common to the conventional device shown in FIG. 4 are denoted by the same reference numerals, and description thereof will be omitted.

<First Embodiment> First, a first embodiment of the present invention will be described using FIG. 1 and FIG. 2.

The apparatus of this embodiment is an electrophotographic apparatus that forms color images.

The color electrophotographic apparatus shown in FIG. 1 has a transfer material conveying system (2) that extends from the right side of the apparatus main body 1 (the right side of FIG. In the section, there is a transfer tram 15 that constitutes the transfer material conveyance system.
The latent image forming section (2) is located close to the latent image forming section (2), and the developing means (i.e., the rotary developing device (2)) is located close to the latent image forming section (2).

First, the transfer material conveyance system described above will be explained.

An opening is formed in the right wall (right side in FIG. 1) of the apparatus main body 1, and removable transfer material supply trays 2 and 3 are disposed in the opening so as to partially protrude outside the machine. There is. Paper feeding rollers 4 and 5 are arranged approximately directly above the trays 2 and 3,
A paper feed roller 6 and a paper feed guide 7.8 are provided so as to communicate these paper feed rollers 4.5 with a transfer tram 15 disposed on the left and rotatable in the direction of the arrow. Near the outer circumferential surface of the transfer tram 15, from the upstream side to the downstream side in the rotational direction, there is a delivery roller 9, a crimper 101, a transfer material separation charger 11. Separation claws 12 are sequentially arranged. In addition, a transfer charger 1 is provided on the inner circumferential side of the transfer tram 15.
3. A charger 14 for separating the transfer material is also provided. A conveyor belt means 16 is disposed at the upper right side of the transfer drum I5 in close proximity to the separation claw 12, and a fixing device 18 is disposed at the (right) end of the conveyor belt means 16 in the transfer material conveyance direction. . A discharge tray 17 is disposed further downstream of the set stone device M18 in the conveying direction, extending outside the apparatus main body 1 and being detachable from the apparatus main body 1.

Next, the configuration of the latent image forming section (2) will be explained. First, the first
Photosensitive tram 19 as an image carrier rotatable in the direction of arrow B in the figure
is disposed with its outer peripheral surface in contact with the outer peripheral surface of the transfer tram I5. Above the photosensitive tram 19, near its outer peripheral surface, from the upstream side to the downstream side in the rotational direction of the photosensitive tram 19, a charger 20 for neutralizing static electricity and a cleaning means 21 are provided.
and a charger 23 are sequentially arranged, and furthermore, an image exposure means 24 such as a laser beam scanner and an image exposure reflection means 25 are arranged for forming an electrostatic latent image on the outer peripheral surface of the L photosensitive tram 19. It is set up.

Next, the structure of the rotary developing device (2) is as follows. A rotatable housing (hereinafter referred to as "rotating body") 26 is disposed at a position facing the outer peripheral surface of the photosensitive tram 19,
Four types of developing devices are mounted in the rotating body zIi at opening positions in the circumferential direction, and are designed to visualize (that is, develop) the electrostatic latent image formed on the outer peripheral surface of the photosensitive tram 19. There is. The above four types of developing devices are a yellow developing device 27Y, a magenta developing device 27M, a cyan developing device 21C, and a fluff developing device 278, respectively.

The entire sequence of the image forming apparatus having the above-mentioned configuration will first be explained in detail using a full color mode as an example. When the photosensitive tram 19 described above rotates in the direction of arrow B in FIG. 1, the photosensitive body of the photosensitive tram 1Lh is uniformly charged by the primary charger 23. - When the photoreceptor is uniformly charged by the charger Z3, the original 28
Image exposure is performed using laser light E modulated by a yellow image signal, and an electrostatic latent image is formed on the photosensitive drum 19, and a yellow developing device 27Y, which has been fixed in advance at a developing position by the rotation of the rotating body 26, performs image exposure. The electrostatic latent image is developed.

On the other hand, the transfer material conveyed via the paper feed guide 7, the paper feed roller 6, and the paper feed guide 8 is held by the clipper 10 at a predetermined timing, and is held by the clipper 10 at a predetermined timing, and is held by the abutment roller 9 and the corresponding contact circular roller 9. The transfer tram 15 is electrostatically wound around the transfer tram 15 by the opposite electrodes. The transfer drum 15 rotates in the direction of arrow A in FIG. The image is transferred by the transfer charger 13 at the portion in contact with the surface. The transfer tram 15 continues to rotate and prepares for transferring the primary color (magenta in FIG. 1).

On the other hand, the photosensitive drum 19 is neutralized by the neutralizing charger 20, cleaned by the cleaning means 21, charged again by the primary charger 23, and subjected to image exposure as described above using the primary magenta image signal. The rotary developing device rotates while an electrostatic latent image is formed on the photosensitive tram 19 by the magenta image signal by the image exposure, and fixes the magenta developing device 27M at the predetermined developing position. Perform magenta development. Subsequently, the process described above was carried out for cyan color and fluff color, respectively, and the transfer of four colors was completed.
Then, the four-color image formed on the transfer material 1 is neutralized by each of the chargers 20 and 14, the transfer material is released from being gripped by the clipper IO, and the transfer material is transferred to the transfer tram by the separation claw 12. 15, and is sent to a fixing device 18 by a conveyor belt 16, where it is fixed by heat and pressure, completing a series of full-color print sequences, and forming the desired full-color print image.

As described above, since a color image consists of three to four layers of multicolor toner, the color toner used has the following characteristics.

The above-mentioned color toner needs to have good melting properties and color mixing properties when heat is applied, and it is preferable to use a sharp-melt toner having a low softening point and low melt viscosity. That is, by using such a sharp melt toner, it is possible to widen the color reproduction range of a copy and obtain a color copy that is faithful to the original image.

Such sharp melt toners are produced by melt mixing, pulverizing, and classifying toner forming materials such as polyester resins, styrene-acrylic resins, colorants (dyes, sublimable dyes), and charge control agents. If necessary, an external addition step of adding various external additives to the toner may be added.

It has good fixing properties for a color toner like this.

In consideration of sharp melt properties, it is particularly preferable to use polyester resin as the binder resin.

Examples of sharp-melt polyester resins include polymeric compounds having an ester bond in the main chain of the molecule synthesized from a diol compound and a dicarboxylic acid.

In particular, the following formula (wherein R is an ethylene or propylene group °C, x,
Each y is a positive integer of 1 or more, and the average value of x+y is 2 to 1O. ) or a substituted product thereof as a diol component, and a carboxylic acid component consisting of a divalent or higher carboxylic acid, its acid anhydride, or its lower alkyl ester (for example, fumaric acid,
Maleic acid, maleic anhydride, phthalic acid, terephthalic acid.

A polyester resin mainly copolymerized with at least wM trimellitic acid, pyromellitic acid, etc. is more preferable because it has sharp melting characteristics.

The softening point of sharp melt polyester resin is 75~
The temperature may be 150°C, preferably 80-120°C.

An example of the softening properties of this sharp melt toner is shown in FIG. The measurement conditions are as follows.

Flow tester (Model: FT-500A (manufactured by Deku Seisakusho)
The diameter of the die (nozzle) is 0.21 and the thickness is 1.0.
Add an extrusion load of 20 kg (ml) and initial set temperature to 70.
℃, and after a preheating time of 300 seconds, a toner plunger drop amount-temperature curve (hereinafter referred to as a softening S-curve) drawn when the humidity was raised at a constant rate of 6° C./min was determined. The sample toner used is 1 to 3 g of purified fine powder, and the plunger cross-sectional area is 1.0 c. The softening S-shaped curve becomes a kerf as shown in Figure 2. As the temperature rises at a constant rate, the toner is gradually heated and begins to flow out (plunger descends A→
B). When the temperature is further increased, the molten toner largely flows out, and B4C-D) the plunger stops descending and ends (D→E).

The height H of the S-curve indicates the total outflow, and the temperature T corresponds to point C of H/2. indicates the softening point of the toner.

Whether or not the toner and the binder resin have sharp melt properties can be determined by measuring the apparent melt viscosity of the toner or the binder resin.

A toner or a binder resin having such sharp melting properties has an apparent melt viscosity of 10:I poise when the temperature is T, and when the temperature when the apparent melt viscosity is 5 x 102 poise is T2, T □ = 90 to 150°C, 1ΔT l = IT, T2+ = 5 to 20°C.

Sharp-melt resins having these temperature-melt viscosity characteristics are characterized by an extremely sharp decrease in viscosity when heated.

Such viscosity reduction causes proper mixing of the top toner layer and the bottom toner layer, and furthermore, the transparency of the toner layer itself increases rapidly, resulting in good subtractive color mixing. Sharp-melt color toner has a high affinity and has the property of being easily offset to the fixing roller.

Therefore, in a fixing device of an image forming apparatus using color toners as described above, a fixing roller that exhibits high releasability over a long period of time is required.

Therefore, in this embodiment, high mold releasability is maintained by using a material with less reduction in oil absorption for the offset prevention layer of the fixing roller. less than.

The results of a durability test using the fixing device of this example will be explained.

First, a method for measuring changes in oil absorption in this example will be explained.

For measurement, a test piece of rubber to be coated on the surface layer of the fixing roller is prepared for testing. The test piece used has a thickness of 21 mm, a horizontal length of 20 s, and a vertical length of 10 mm.

The measurement is performed using the following oil absorption rate change measurement method.

First, the initial oil absorption rate is measured and this oil absorption rate is defined as A%.

Next, the oil absorption rate of the test piece heated by placing it in an oven at 200° C. for 1000 hours was measured, and this oil absorption rate was defined as B%.

Then, the difference between A and B is calculated.

C (%) = B - A This C is the difference in oil absorption rate from the initial stage and represents the change in oil absorption rate.

When the oil absorption rate change of the fixing roller explained in the conventional example was measured using the above method (hereinafter referred to as oil absorption rate change measurement method), it was found that
-10%. When the oil absorption rate decreases in this way, the uptake of oil into the surface layer becomes poor, and the mold release effect from inside the surface layer decreases. Therefore, it is considered that a sufficient mold release effect could not be obtained by pouring oil from the outside, and offset occurred.

Therefore, in this example, as a method of suppressing the decrease in oil absorption rate, the amount of filler was reduced by 10% from the amount recommended by the manufacturer. The amount recommended by the manufacturer of the filler or cross-linking agent is set in such a way that the rubber does not soften or become brittle when heated, or if there is too much of the filler or cross-linking agent, the oil absorption rate will decrease. Therefore, it is possible to prevent a decrease in oil absorption by reducing the amount of filler or crosslinking agent to an extent that does not cause any problems in terms of durability and strength. - As an example, - liquid-based RTV silicone rubber was mixed with a filler in an amount reduced by 10% from the recommended amount, and then cured. As a result, in the above oil absorption rate change measurement method, initial 40%,
After the heating test, the oil absorption rate was 40%, and the oil absorption rate change was 0%. Further, when a fixing test was conducted using this material, good fixing was possible with no offset occurring even when fixing 10,000 sheets.

(Second example) Next, a second embodiment of the present invention will be described.

Note that the same reference numerals are given to the same parts as in the first embodiment, and the explanation thereof will be omitted.

This example differs from the first example in that the anti-offset layer is made of a liquid RTV silicone rubber coated with 10 parts of 100 C3 dimethyl silicone oil and cured.

This rubber piece was found to have an oil absorption rate of 40% at the initial stage and 50% after the heating test, resulting in an oil absorption rate change of +10%.

When we conducted a fixation test using this material, l
Even when fixing 00,000 sheets, good fixing was possible without occurrence of offset.

(Comparative Example) Next, for comparison, an experimental example conducted using a conventional fixing roller will be described.

This comparative example differs from the second example in that LTV silicone comb (low-temperature vulcanizable silicone rubber) is used as the anti-offset layer and cured by heating.

In this rubber piece, the oil absorption rate change was measured to be 25% initially and 15% after the heating test, and the oil absorption rate change was -IO%.

When we conducted a fixation test using this material, 2
Offset occurred after 10,000 copies.

(Third example) Next, a third embodiment of the present invention will be described.

Note that the same reference numerals are given to the same parts as in the first embodiment, and the explanation thereof will be omitted.

This example is different from the first example in that the anti-offset layer is coated with a material in which 5 parts of 10-mouth C8 dimethyl silicone oil is added to LTV silicone rubber and cured.

The oil absorption rate change of this rubber piece was 30% at the initial stage and 25% after the heating test, and the oil absorption rate change was 15%.

When we conducted a fixation test using this material, 1
Good fixing was achieved with no offset occurring after 0,000 copies.

<Fourth Example> Next, a fourth embodiment of the present invention will be described.

Note that the same reference numerals are given to the same parts as in the first embodiment, and the explanation thereof will be omitted.

This example differs from the first example in that LTV silicone rubber is used as the offset prevention layer, and the cross-linking agent of this material is reduced by 50% compared to the manufacturer's recommendation. .

In this rubber piece, the oil absorption rate change was measured to be 60% initially and 75% after the heating test, and the oil absorption rate change was +[5%.

When a fixation test was conducted using this material, lO
Good fixing was possible with no offset occurring up to 10,000 sheets.

<Fifth Embodiment> Next, a fifth embodiment of the present invention will be described using FIG. 3. Note that descriptions of parts common to the first embodiment will be omitted.

In this embodiment, an elastic layer 3 made of FfTV silicone rubber on a core bar 29 is used as a fixing roller, as shown in FIG.
This embodiment differs from the first embodiment in that a fluorine comb layer F having a thickness of 1 OBm is formed as an oil barrier layer between the offset prevention layer 31 and the offset prevention layer 31.

As the material of the offset prevention layer 31, 'LTV
Using silicone rubber, the material contains too cs
A film was coated with 30 parts of dimethyl silicone oil added thereto, and the film was cured by heating.

The oil absorption rate change of this rubber was 50% at the initial stage and 65% after the heating test, and the oil absorption rate change was +15%.

When a fixation test was conducted using this material, lO
After 10,000 copies, no offset occurred and good fixing was achieved.

As described above, the configuration in which the intermediate layer includes an oil barrier layer such as fluororubber prevents the oil retained in the offset prevention layer from further penetrating into the lower layer, allowing the oil to be effectively utilized. However, oil from the outside and oil from the inside can be effectively used to prevent offset.

Table 1 summarizes the embodiments described above.

(Hereinafter in the margin) Table 1 (hereinafter in the margin) As mentioned above, it was found that offset can be prevented if the oil absorption rate changes by 15% or more. Also, oil absorption rate change is 0%
The above is preferable. This is because considering that the surface layer absorbs oil, it is better to maintain its oil absorption rate in its initial state, and this is better from the standpoint of stable toner release.

Additionally, there is a concern that if the rubber has an oil absorption rate of less than 15%, it may deteriorate when used under heat, but experiments have shown that this phenomenon does not progress significantly and is within the range of no practical problems. there were. This is because the conventional processing methods recommended by rubber material manufacturers contain more fillers, crosslinking agents, etc. than necessary, and the rubber is made with sufficient strength even with slight changes in processing methods. In this respect, even if the amount of filler and crosslinking agent is slightly reduced from the manufacturer's recommended amount as in the present invention, it is thought that no problem will occur in terms of durable strength.

Incidentally, problems such as heat deterioration may be improved by using a heat resistance improver as a conventionally known filler.

Further, in the present invention, the amount of the release agent applied in the fixing device is not particularly limited, and is preferably 0.001 g.
/A4 or larger is good. This is because if it is less than this, the amount of oil supplied from the outside will be insufficient, and the oil retention effect of Offsera 1 to the prevention layer will be weakened.

Here, a method for measuring the coating amount of the mold release agent will be explained.

First, the weight of 50 sheets of A4 size white paper is assumed to be A+ (g), and the transfer of the image onto the white paper is also zero.The offset protection layer of the fixing roller is also coated with silicone oil. Blank paper 5 after passing between the paper and the pressure roller
Let the weight of 0 sheets be B (g). Next, another A4 in the same way
Let the weight of 50 sheets of white paper be A2 (g), and either do not transfer the image onto this white paper, or apply silicone oil to the offset prevention layer of the fixing roller, and make sure that the fixing roller and pressure roller are not in contact with each other. Assuming that the weight of 50 sheets of blank paper after passing through the gap is c (g), the amount of silicone oil applied per sheet of A4 size blank paper X (g/A4) can be determined as follows.

X= (C+A+B A2) 150 The above value of 0.0 old g/A4 was calculated based on the above formula.

The offset prevention layer is shown as an example of a rubber having a somewhat high initial oil absorption (30% or more), but the initial oil absorption may be any value, preferably 20% or more. This is because, if the toner does not have a certain degree of oil absorption at the beginning, for example, in the case of a toner with poor releasability such as a color toner, offset will occur from the beginning of use.

Also, the thickness of the anti-offset layer is not particularly limited.
Preferably 10. μ■ or higher is good. This is because if it is too thin, even if the oil absorption rate is high, the amount of oil that can be held within the offset prevention layer will be small, and a large mold release effect will not be obtained. In addition, if it is thin, uneven coating of the offset prevention layer tends to occur during roller molding, which is not preferable.

Note that the present invention is effective not only in color image forming apparatuses but also in fixing devices in monochrome image forming apparatuses.

In addition, in the examples, whether the amount of rubber filler or crosslinking agent was used in a smaller amount than the manufacturer's recommended amount.1 In the present invention, the absolute amount of the manufacturer's recommended amount is not a problem, but the change in oil absorption after heat curing. This is proposed as a method for increasing the amount to 15% or more.

[Effects of the Invention] As explained above, according to the present invention, the offset prevention layer on the surface layer of the fixing rotor has an oil absorption rate change of 15% or more as measured by the oil absorption rate change measurement method, so it can prevent offset over a long period of time. It is possible to prevent the occurrence of In particular, even when using color toners that tend to cause offset, it is possible to provide good images over a long period of time without causing offset.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing the schematic configuration of the apparatus according to the first embodiment of the present invention, FIG. 2 is a diagram showing the relationship between plunger fall amount and temperature of the sharp melt toner used in the apparatus shown in FIG. 1, and FIG.
The figure is a sectional view showing a schematic configuration of a device according to a fifth embodiment of the present invention.
FIG. 4 is a sectional view showing a schematic configuration of a conventional device.

Claims (1)

[Scope of Claims] A fixing device that is equipped with a fixing rotary body having a heating element therein and fixes an unfixed developer image on a recording material, wherein the fixing rotary body has an offset prevention layer on its surface layer; A fixing device characterized in that the change in oil absorption of the offset prevention layer is set to a value measured by an oil absorption change measurement method of -5% or more.