JP2010227859A - Coating device and method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a coating device and method in which any roping and blur are not generated for a strip continuously traveling at a high speed and in which the appearance of a coated film is not spoiled. <P>SOLUTION: A pick up roll 4 pumping up a coating liquid 3 filled in a coater pan 2, an applicator roll 6 transferring to the strip S the coating liquid 3 pumped up by the pick up roll 4, and an ultraviolet irradiation device 9 irradiating the surface of the strip S with the ultraviolet ray before the coating liquid 3 being transferred to the strip S by the applicator roll 6 are included in the coating device. The ultraviolet ray radiated at a distance of 0.01-200 mm by the ultraviolet irradiation device 9 possesses at least waves of 40 or longer and shorter than 240 nm, and waves of 240 or longer and 400 nm or shorter. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a coating apparatus and a coating method for performing a coating process continuously on a long strip such as a steel plate using a roll coater.

  Conventionally, a coating film is formed by applying a coating liquid having various physical properties on the surface of a strip such as a steel plate that continuously travels, and the strip has performance such as corrosion resistance, workability, aesthetics, and insulation. The process to give to is performed. A roll coater is generally used as a coating apparatus for performing this treatment method. Examples of the roll coater include a two-roll coater having two rolls and a three-roll coater having three rolls. In particular, the three-roll coater is used as a mainstream coating apparatus because it has excellent controllability of the film thickness of the coating film and can finish the appearance of the surface of the coated object relatively beautifully.

  FIG. 2 is a schematic diagram showing the configuration of a conventional three-roll coater. As shown in FIG. 2, the conventional roll coater 1 includes a coater pan 2 filled with a coating liquid 3, a pickup roll 4, a metering roll 5, and an applicator roll 6. The coating liquid 3 drawn up from the coater pan 2 by the pick-up roll 4 is adjusted in the amount of liquid by the mitering roll 5 and applied to the peripheral surface of the applicator roll 6. The coating liquid 3 applied to the peripheral surface of the applicator roll 6 is transferred to the band S when the applicator roll 6 contacts the band S traveling in a predetermined direction. The direction of rotation of each of the rolls 4, 5, 6, and 8 is reverse rotation that rotates in the opposite direction to the case of natural rotation that rotates in the same direction at the close point or close point between the rolls 4, 5, 6, and 8. There is a case. Among these rotation directions, in general, the reverse rotation makes it easier to obtain a relatively smooth coating film surface, and therefore, particularly between the applicator roll 6 and the strip S, the reverse rotation is performed. There are many cases to do. The applicator roll 6 uses a rubber roll in which rubber is lined on a steel roll so as not to damage the surface of the band S.

  However, as a typical application defect of the application method using the three-roll coater, a streak pattern (hereinafter referred to as roping) generated along the circumferential direction of the applicator roll 6 is transferred to the band S, resulting in deterioration in appearance. There is. Roping tends to occur more easily as the viscosity of the coating solution is higher. Further, roping tends to occur more easily as the peripheral speed of the roll becomes higher. In particular, when the traveling speed of the strip S (hereinafter referred to as the line speed) is increased, the peripheral speed of each roll is also increased, so that it is difficult to prevent the occurrence of roping.

Thus, techniques described in Patent Documents 1 to 3 are disclosed as techniques for preventing the occurrence of roping.
Patent Document 1 discloses a technique for controlling the ratio of the line speed, the peripheral speed of the applicator roll, and the peripheral speed of the pickup roll to a specific range.
Patent Document 2 discloses a technique for maintaining the temperature of a paint (coating liquid) near the temperature at which the viscosity of the paint is minimized.

Further, Patent Document 3 discloses a technique for reducing the occurrence of roping in high-speed coating on a steel plate having a line speed of 200 mpm or more. Specifically, it is a technique for performing application with a peripheral speed of the pickup roll of 20 to 80 mpm and a peripheral speed of the applicator roll of 200 to 1000 mpm which is equal to or higher than the line speed.
Further, in recent years, with increasing needs for processability and high electrical conductivity, it has been required to reduce the thickness of the coating film (thickness after coating and drying of less than 0.1 μm). As a thin film coating method, a micro gravure method using a small-diameter gravure roll (see Patent Document 4) and a die coater method are disclosed (see Patent Document 5). In the microgravure method, the coating film thickness can be reduced to less than 3 μm and a minimum of about 1 μm, and in the die method, the coating can be performed to about 1 μm.

JP 2000-254580 A JP 9-47716 A JP-A-10-309512 Japanese Patent Laid-Open No. 11-197569 JP 2004-230361 A

However, with the techniques disclosed in Patent Document 1 and Patent Document 2, it is possible to prevent the occurrence of roping if the line speed is low, but it becomes difficult to prevent the occurrence of roping if the line speed becomes high. Therefore, since the line speed at which a uniform appearance without occurrence of roping can be obtained is limited to less than 200 mpm, the yield is lowered.
In the technique disclosed in Patent Document 3, when the peripheral speed of the applicator roll is increased, the roping pitch on the applicator roll becomes finer, and the roping is less noticeable on the steel plate.

  However, if the peripheral speed of the applicator roll is increased too much with respect to the line speed, the flow of the coating liquid is disturbed at the position where the applicator roll and the steel sheet are in contact with each other, and the interface of the coating liquid is locally oscillated in the width direction. A mottled pattern is formed on the top coating. Therefore, increasing the peripheral speed of the applicator roll is an effective means for preventing the occurrence of roping, but has a problem of newly creating factors that impair the appearance of the coating film, such as a decrease in smoothness.

Further, in the techniques described in Patent Document 4 and Patent Document 5, in the case of the micro gravure method, the coating liquid is transferred to the substrate using a metal roll. There was a problem that wrinkles occurred on the surface of the material.
In the case of the die coater method, in order to reduce the film thickness, it is necessary to bring the gap between the base material and the die for supplying the coating liquid close to a desired film thickness. In other words, a smooth base material such as a glass substrate can be approached, but in the case of a continuously traveling steel sheet, there is a problem that the die is difficult to approach because shape variation occurs in both the width direction and the longitudinal direction. It was. Further, in the die coater system, when high speed application is performed, a defect called air entrainment occurs, so that the application is limited to a low speed region of about 10 mpm, and productivity is hindered.

In addition, when applying to a steel sheet in a roll coater, the liquid film thickness after application is generally about 5 to 10 μm, and in order to make the film thickness after application and drying less than 0.1 μm, it is necessary to increase the dilution rate There is. In particular, when an aqueous coating solution is used, the surface tension is increased by increasing the dilution rate, so that the wettability with the steel sheet is deteriorated, and scumming is likely to occur.
Accordingly, the present invention has been made in view of the above-described problems, and the object thereof is to improve the wettability and generate the blur even when the dilution rate is increased with respect to a strip that continuously runs at a high speed. An object of the present invention is to provide a coating method using a roll without impairing the appearance of the coating film.

In order to solve the above-described problem, the coating apparatus according to claim 1 of the present invention includes a pickup roll that draws a coating liquid filled in a coater pan on a peripheral surface, and a pickup roll that is close to the pickup roll, In an application apparatus having an applicator roll that adheres the liquid to the peripheral surface and transfers the application liquid to a strip that continuously runs in a predetermined direction.
Before the coating liquid is transferred to the applicator roll, the surface of the strip is irradiated with ultraviolet rays including at least a wavelength of 40 nm or more and less than 240 nm and a wavelength of 240 nm or more and 400 nm or less at a distance of 0.01 mm to 200 mm. An ultraviolet irradiation device is provided.

In addition, in the coating method according to claim 2 of the present invention, the coating liquid filled in the coater pan is attached to the peripheral surface of the rotating pickup roll, and is pumped up close to the pickup roll. In the coating method of transferring the coating liquid to a strip that continuously travels in a predetermined direction by being attached to the peripheral surface,
The speed ratio Va / Vs between the peripheral speed Va of the applicator roll and the traveling speed Vs of the belt-shaped body is 0.7 or more and 1.4 or less, and the peripheral speed Vp of the pickup roll and the peripheral speed Va of the applicator roll The band-shaped body includes an ultraviolet ray including at least a wavelength of 40 nm or more and less than 240 nm and a wavelength of 240 nm or more and 400 nm or less before the coating liquid is transferred to the applicator roll. It is characterized by irradiating the surface.

  According to the coating apparatus according to the first aspect of the present invention, before transferring the coating liquid onto the belt-like body by the applicator roll, ultraviolet light including at least a wavelength of 40 nm or more and less than 240 nm and a wavelength of 240 nm or more and 400 nm or less, By pre-irradiating the surface of the band-shaped body at a distance of 0.01 mm to 200 mm, the wettability of the surface of the band-shaped body irradiated with ultraviolet rays is improved. Therefore, it is possible to provide a coating method using a roll that does not cause scumming even when the dilution rate is increased and does not impair the appearance of the coating film.

  In the coating method according to claim 2 of the present invention, before the coating liquid is transferred to the band by the applicator roll, the ultraviolet ray including at least a wavelength of 40 nm or more and less than 240 nm and a wavelength of 240 nm or more and 400 nm or less Since the wettability of the surface of the band-shaped body irradiated with ultraviolet rays is improved, a good appearance without blur can be obtained even when the coating film is a thin film.

It is the schematic which shows the structure of the coating device used for one Embodiment of the coating method which concerns on this invention. It is the schematic which shows the structure of the conventional coating device.

  Hereinafter, an embodiment of a coating method using a roll coater according to the present invention will be described with reference to the drawings. FIG. 1 is a schematic view showing a configuration of a coating apparatus used in an embodiment of a coating method using a roll coater according to the present invention. As shown in FIG. 1, a roll coater 1 as a coating device includes a coater pan 2 filled with a coating solution 3, three rotating bodies (roll rolls) including a pickup roll 4, a metering roll 5, and an applicator roll 6. ).

  The coating liquid 3 is pumped up from the coater pan 2 by being applied to the peripheral surface of the rotating pickup roll 4. The peripheral surface of the pickup roll 4, the peripheral surface of the mitering roll 5, and the peripheral surface of the applicator roll 6 are close to each other with a predetermined gap. The coating liquid 3 applied to the peripheral surface of the pickup roll 4 is adjusted to a coating liquid amount based on the predetermined gap by the mitering roll 5 and transferred to the peripheral surface of the applicator roll 6. Then, the coating liquid 3 transferred to the peripheral surface of the applicator roll 6 is transferred to the band 1 by the applicator roll 6. The pickup roll 4 uses a gravure roll in which a large number of recesses are formed on the surface of the body portion. The belt-like body 1 travels around the backup roll 8 installed on the opposite side of the applicator roll 6 via the belt-like body 1.

  The rotation direction of each roll is the reverse direction between the rolls and between the applicator roll 6 and the strip 1. A blade 7 that scrapes the coating solution 3 is installed on the mitering roll 5. When the strip 1 is a metal plate such as a steel plate, a rubber roll lined with rubber is used as the applicator roll 6. The belt-like body 1 is sandwiched between the applicator roll 6 and the backup roll 8, and the coating liquid 3 is applied to one side by the applicator roll 6. A configuration in which the coating liquid 3 is applied may be employed.

  Further, on the upstream side of the applicator roll 6, an ultraviolet irradiation device 9 that irradiates the surface of the band S with ultraviolet rays is provided. The ultraviolet irradiation device 9 includes, for example, a cooling device, a power supply device, a light source, and an irradiator (not shown). The ultraviolet rays irradiated from the ultraviolet irradiation device 9 include a plurality of different wavelengths in the range of 40 nm to 240 nm, and include at least a wavelength of 40 nm or more and less than 240 nm and a wavelength of 240 nm or more and 400 nm or less. Moreover, the ultraviolet irradiation device 9 is arranged so that the irradiation distance d from the irradiation port 9a to the surface of the strip S is within a range of 0.01 mm to 200 mm.

  Here, in a conventional roll coater (see FIG. 2) that is not provided with an ultraviolet irradiation device that irradiates the surface of the band S with ultraviolet rays, when a coating solution diluted to a solid content concentration of less than 2% is applied. Since the surface tension is increased and the wettability is deteriorated, the blur is likely to occur in the traveling direction. The cause of the blur is that liquid wetting on the band S is likely to occur due to deterioration of wettability between the band S and the coating liquid.

  Therefore, in order to obtain a uniform film thickness, it is only necessary that the air accompanying the band S does not affect the meniscus between the applicator roll and the band S. Therefore, in this embodiment, by installing the ultraviolet irradiation device 9, the surface of the band S is modified, and it becomes possible to make contact with the applicator roll with good wettability.

In the present embodiment, the coating solution 3 is an aqueous solution containing siri force, a phosphoric acid compound, Mg and a tetravalent vanadium compound, but is not particularly limited. Without being limited to the inorganic component, for example, an emulsion resin-based aqueous solution or the like can be suitably used.
Here, when the coating solution 3 is a coating solution using an aqueous solvent, if the coating solution 3 is diluted too much, the wettability of the strip S such as a steel plate deteriorates as the surface tension increases, and Will occur. That is, when the concentration of the coating solution is 1%, the surface tension becomes almost equal to that of water, and repelling occurs. Therefore, in order to ensure wettability to the belt-like body S such as a steel plate, the concentration of the coating solution 3 is preferably 3% or more.

Next, the conditions under which the wettability is improved by the ultraviolet irradiation of the belt S by the ultraviolet irradiation device 9 will be described.
The improvement of the wettability of the band S is manifested by the generation of functional groups such as CO and CO (OH) on the surface of the band S. The production of CO and CO (OH) requires a reaction between an organic compound and active oxygen. It is known that ultraviolet rays react with oxygen in the air in the wavelength region of 240 nm or less to generate ozone. The ozone is decomposed by ultraviolet energy to generate active oxygen.

However, even if ultraviolet rays having a short wavelength of 240 nm or less are included, improvement in wettability was not observed when visible light having other wavelengths was 400 nm or more. This is thought to be because the wavelength increased and the energy of ultraviolet rays decreased, so that the energy of ozonolysis became insufficient, and active oxygen having high energy was not generated.
In addition, in the wavelength range of less than 40 nm, no improvement in wettability was observed. This seems to be because there was much ultraviolet absorption in the air and no active oxygen was generated. Accordingly, the wavelength of the ultraviolet light is in the range of 40 to 400 nm.

Moreover, wettability was not improved only by ultraviolet rays having a wavelength of 240 nm or less. It seems that the ultraviolet ray absorption of oxygen is large and the irradiation intensity is lowered, so that sufficient energy for ozonolysis cannot be obtained. Therefore, the wavelength types are set to two or more types including 40 nm or more and less than 240 nm and 240 nm or more and 400 nm or less.
Further, when the distance (irradiation distance) between the irradiation port 9a of the light source of the ultraviolet irradiation device 9 and the surface of the band S was increased, no improvement in wettability was observed. It seems that a lot of ultraviolet rays were absorbed by oxygen. Therefore, the irradiation distance was set to 200 mm or less where improvement of wettability was observed. When the irradiation distance was less than 0.01 mm, a sufficient amount of active oxygen was not obtained and no improvement in wettability was observed. Moreover, when the strip | belt-shaped body S has dispersion | variation in thickness, there exists a danger that a light source and the strip | belt-shaped body S will contact. Therefore, the range of the irradiation distance is set to 0.01 mm or more and 200 mm or less.

Next, consideration will be given to the peripheral speed condition of each roll that makes the appearance uniform after coating the coating liquid.
Roping is more likely to occur as the viscosity of the coating liquid 3 is higher. Further, roping tends to occur more easily as the peripheral speed of the roll, particularly the applicator roll 6 is higher. The reason for the occurrence of roping is thought to be that the air flow accompanying the band S disturbs the meniscus that bridges between the band S and the applicator roll 6. That is, the conditions for the occurrence of roping mainly depend on the peripheral speed of each roll, the pressing pressure between a plurality of rolls, and the physical properties (viscosity and surface tension) of the coating liquid 3. In particular, in the coating process in which the line speed Vs is increased (hereinafter referred to as “high-speed coating”), the peripheral speed Va of the applicator roll 6 is increased in accordance with the line speed Vs, and the peripheral speed of each roll is inevitably increased. Therefore, the occurrence of roping is difficult to avoid.

  The roping may occur when the coating liquid 3 is transferred between the mitering roll 5 and the pickup roll 4 and when the coating liquid 3 is transferred between the pickup roll 4 and the applicator roll 6. When the high-speed application is performed, the latter always occurs. Further, when roping occurs in the latter, since the roping generated in the former is canceled, it is only necessary to pay attention to the transfer of the coating liquid 3 between the pickup roll 4 and the applicator roll 6.

  As a result of the inventors conducting various experiments on these, as the peripheral speed Vp of the pick-up roll 4 is increased, the roping pitch λ on the applicator roll 6 is decreased, and the peripheral speed Vp of the pick-up roll 4 is decreased. With respect to the peripheral speed Va of the applicator roll 6, a very fine wavenumber roping was obtained at 0.5 times or more. When the roping pitch λ is small, uneven coating may become inconspicuous, but the leveling effect increases after application to drying, so that the appearance of the coating film surface after drying is improved. Here, the upper limit of the ratio (Vp / Va) between the peripheral speed Vp of the pickup roll 4 and the peripheral speed Va of the applicator roll 6 is preferably 2.0 or less. When Vp / Va exceeds 2.0, the supply of the coating liquid 3 from the pickup roll 4 becomes excessive, and a coating defect called a ripple defect is caused by a meniscus that bridges between the applicator roll 6 and the pickup mouth 4. It becomes easy to generate newly.

  Further, the peripheral speed Va of the applicator roll 6 is set to 0.7 times or more and 1.4 times or less with respect to the line speed Vs of the band S. The relationship between the peripheral speed Va of the applicator roll 6 and the line speed Vs of the strip S is the same as the relationship between the peripheral speed Vp of the pickup roll 6 and the peripheral speed Va of the applicator roll 6. The lower the peripheral speed Va of the applicator roll 6 with respect to the line speed Vs, the more unstable the flow of the meniscus portion due to the effect of air brought in from the steel plate, and thus the roping appears more remarkably. When the peripheral speed Va of the applicator roll 6 is increased and the ratio of the peripheral speed Va of the applicator roll 6 to the line speed Vs (Va / Vs) is 0.7 or more, roping is reduced and a good appearance is obtained. Is obtained.

  However, when the peripheral speed Va of the applicator roll 6 is excessively increased, the appearance is impaired. The reason for this is considered to be that when the peripheral speed Va of the applicator roll 6 is excessively increased, roping that occurs due to transfer of the coating liquid between the pickup roll 4 and the applicator roll 6 cannot be avoided. That is, when only the space between the applicator roll 6 and the belt-like body S is considered, a coating film with a better appearance can be obtained when the peripheral speed Va of the applicator roll 6 is higher, but between the pickup roll 4 and the applicator roll 6 Then, roping is likely to occur, and the roping generated in the applicator roll 6 is transferred to the strip as it is.

In order to avoid roping between the pickup roll 4 and the applicator roll 6, as described above, the peripheral speed Vp of the pickup roll 4 may be increased. However, the peripheral speed Va of the applicator roll 6 is excessively high. If it is too high, the peripheral speed Vp of the pickup roll 4 must be very high.
However, if the peripheral speed Vp of the pick-up roll 4 is increased too much, the coating liquid 3 is scattered significantly when the coating liquid 3 is drawn from the coater pan 2, or the wear of the mitering roll 5 or applicator roll 6 progresses. Is unfavorable for actual operation.

  In view of the above problems, in this embodiment, in addition to the ultraviolet irradiation to the surface of the band S by the ultraviolet irradiation device, the peripheral speed Va of the applicator roll 6 may be 1.4 times or less the line speed Vs. preferable. When the peripheral speed ratio (Va / Vs) of the peripheral speed Va of the applicator roll 6 to the line speed Vs exceeds 1.4 times, the liquid pool of the coating liquid 3 between the strip S and the applicator roll 6 vibrates. It is easy to cause uneven coating.

(Example)
Examples of the coating method according to the present invention and comparative examples will be described in detail.
Application is performed on the coiled strip S of a galvanized steel sheet having a plate thickness of 0.85 mm and a plate width of 1000 mm using the coating apparatus 1 shown in FIG. The coated strips S of 1 to 15 and Comparative Examples 1 to 12 were obtained. With respect to the coated strip S, the coating appearance after drying, the film thickness of the coating film after coating, and the surface tension of the coating film were measured. The evaluation results and the measurement results are shown in Table 1.

  In the present example and the comparative example, a rubber roll lined with rubber was used as the applicator roll 6 in the coating apparatus 1 shown in FIG. Further, as the pickup roll 4, a gravure roll having a large number of concave portions formed on the surface thereof was used. A rubber roll lined with rubber was used as the mitering roll 5. The roll diameters of the applicator roll 6 and the pickup roll 4 are 300 mm. The roll diameter of the mitering roll 5 is 200 mm.

As the ultraviolet irradiation device 9, a device having a control device capable of dimming the ultraviolet lamp was used in order to change the wavelength.
The used coating solution 3 was one having a viscosity of 3 (mPa · s), a surface tension of 40 dyn / cm, and a solid content concentration of 5% at a liquid temperature of 20 ° C., further diluted with water.
The appearance after the application was evaluated by visual observation under a sufficiently bright fluorescent lamp based on the following evaluation criteria.

As evaluation of wettability, the surface tension of the coating film was obtained by measuring static and dynamic contact angles using a contact angle meter PG-x (manufactured by Matsubo).
In Examples 1 to 15 and Comparative Examples 1 to 11, coater conditions that do not cause roping were selected and evaluated.
[Evaluation criteria for damage]
◎: No blurring ○: There is little blurring △: There is a slight blurring ×: There are many blurrings

  As shown in Table 1, in Examples 1 to 15, the band-shaped body is irradiated with ultraviolet rays having different wavelengths at a predetermined distance from 240 nm as a boundary, thereby suppressing the occurrence of blurring and forming a coating film uniformly. It was. In particular, in Examples 6 to 12, 14, and 15, the reduction in blurring was good.

  On the other hand, Comparative Examples 1 and 2 in which the wavelength of the irradiated ultraviolet rays was not set to a distance of 0.01 mm to 200 mm, the wavelength of the irradiated ultraviolet rays did not include wavelengths of 40 nm to less than 240 nm and wavelengths of 240 nm to 400 nm. In Comparative Examples 3 to 10 and Comparative Example 11 in which ultraviolet rays were not irradiated in advance, the occurrence of scumming was significant.

  As described above, according to the coating apparatus and the coating method according to the present invention, before transferring the coating liquid to the belt-like body, the ultraviolet ray including at least a wavelength of 40 nm or more and less than 240 nm and a wavelength of 240 nm or more and 400 nm or less is used. By previously irradiating the surface of the band-like body at a distance of 0.01 mm to 200 mm, it is possible to prevent the occurrence of blurring and to form a coating film uniformly.

  As mentioned above, although embodiment of this invention has been described, this invention is not limited to this, A various change and improvement can be performed. For example, although the cold rolled steel plate is used as the belt in the above embodiment, the present invention is not particularly limited to the steel plate, and other metal plates such as aluminum may be used.

DESCRIPTION OF SYMBOLS 1 Coating device 2 Coater pan 3 Coating liquid 4 Pickup roll 5 Mitering roll 6 Applicator roll 7 Blade 8 Backup roll 9 Ultraviolet irradiation device 9a Irradiation port S Band

Claims (2)

A pick-up roll that draws the coating liquid filled in the coater pan on the peripheral surface, and a belt-like body that is close to the pickup roll, adheres the coating liquid to the peripheral surface, and continuously travels in a predetermined direction. In an applicator having an applicator roll for transferring the coating liquid to
Before the coating liquid is transferred to the applicator roll, the surface of the band-shaped body is irradiated at a distance of 0.01 mm to 200 mm with ultraviolet rays including at least a wavelength of 40 nm to less than 240 nm and a wavelength of 240 nm to 400 nm. A coating apparatus characterized in that an ultraviolet irradiation device is provided. The coating liquid filled in the coater pan adheres to the peripheral surface of the rotating pick-up roll, draws up, adheres to the peripheral surface of the rotating applicator roll close to the pick-up roll, and continuously travels in a predetermined direction. In the coating method of transferring the coating liquid to the belt-shaped body
The speed ratio Va / Vs between the peripheral speed Va of the applicator roll and the traveling speed Vs of the belt-shaped body is 0.7 or more and 1.4 or less, and the peripheral speed Vp of the pickup roll and the peripheral speed Va of the applicator roll The band-shaped body includes an ultraviolet ray including at least a wavelength of 40 nm or more and less than 240 nm and a wavelength of 240 nm or more and 400 nm or less before the coating liquid is transferred to the applicator roll. A coating method characterized by irradiating the surface of the coating.

Images