KR20190101703A - Composition for porous sheet which has good shock-absorptivity, method for irradiation cross-linked porous sheet and irradiation cross-linked porous sheet manufactured therefrom - Google Patents

Abstract

The present invention relates to a composition for porous sheets excellent in shock absorption, a method for manufacturing an electron beam crosslinked porous sheet, and an electron beam crosslinked porous sheet manufactured therefrom. The composition comprises: 10-80 parts by weight of a base resin including any one or a mixture of two or more thereof selected from a group consisting of a thermoplastic polyolefin resin, a thermoplastic polystyrene resin, thermoplastic rubber, a silicone-based resin, a fluorine-based resin, and an engineering plastic; 20-90 parts by weight of a plasticizer; and 0.1-5.0 parts by weight of a crosslinking agent.

Description

Composition for porous sheet which has good shock-absorptivity, method for irradiation cross-linked porous sheet and irradiation cross-linked porous sheet manufactured therefrom}

The present invention relates to a composition for a porous sheet, a method for producing an electron beam crosslinked porous sheet, and an electron beam crosslinked porous sheet prepared therefrom, and more particularly, a composition for a porous sheet excellent in shock absorption, a method for producing an electron beam crosslinked porous sheet, and prepared therefrom. It relates to an electron beam crosslinked porous sheet.

Electronic devices such as mobile phones, hard disk drives (HDDs), televisions and liquid crystal displays are made up of precise mechanical components and electronic devices. The electronic devices are easily broken or broken when a physical shock is applied from the outside, and contaminants such as dust are introduced from the outside to disturb the air flow in the electronic device, causing overheating of the electronic device, and shortening the lifespan.

In order to solve these problems, in general, electronic devices are provided with a sheet that serves as a sealing (sealing) that can absorb the impact from the outside and close the gap of the exterior.

For example, Korean Patent Laid-Open Publication No. 2002-0015239 prevents optical sheets from being damaged or wrinkled by a guide panel of a liquid crystal display device, maintains a gap between the panel guide and the light guide plate, and maintains a gap between them. It mentions the installation of a separate space keeping / heat blocking member to block the inflow of foreign substances. In this case, a pad made of silicon is used as the gap maintaining / heat blocking member.

In addition, Korean Laid-Open Patent Publication No. 2005-0058055 discloses forming a separate elastic layer between substrates in order to prevent gravity failure of the liquid crystal display device. In this case, the elastic layer is prepared by preparing a paste containing an aromatic diisocyanate, polyol and 1,4-butanediol, coating on a substrate, and then curing to form a polyurethane rubber layer.

However, these materials have a problem in that the shock absorbency is not relatively excellent, which is a problem.

The problem to be solved by the present invention is to provide a composition for porous sheets excellent in shock absorption, a method for producing an electron beam crosslinked porous sheet and an electron beam crosslinked porous sheet prepared therefrom.

According to an aspect of the present invention, a base resin comprising any one or a mixture of two or more thereof selected from the group consisting of a thermoplastic polyolefin resin, a thermoplastic polystyrene resin, a thermoplastic rubber, a silicone resin, a fluorine resin, and an engineering plastic To 80 parts by weight; 20 to 90 parts by weight of a plasticizer; And 0.1 to 5.0 parts by weight of the crosslinking aid is provided.

Here, the plasticizer is liquid at room temperature, and may be any one or a mixture of two or more selected from the group consisting of a paraffinic plasticizer, an olefin plasticizer, a naphtha plasticizer, and an aromatic plasticizer.

The crosslinking aid may be any one selected from the group consisting of vinyl monomers, acrylate compounds, methacrylate compounds, and epoxy compounds, or a mixture of two or more thereof.

According to another aspect of the present invention, (S1) comprising a thermoplastic polyolefin resin, thermoplastic polystyrene resin, thermoplastic rubber, silicone resin, fluorine resin, any one selected from the group consisting of engineering plastics or a mixture of two or more thereof 10 to 80 parts by weight of basic resin; 20 to 90 parts by weight of a plasticizer; And kneading 0.1 to 5.0 parts by weight of the crosslinking aid to prepare a raw material composition. (S2) inserting the raw material composition into an extruder to extrude; (S3) casting the result of the step (S2); (S4) forming pores by dipping the resultant of the step (S3) in a solvent to extract the plasticizer; And (S5) after drying the product of the step (S4), there is provided a method for producing an electron beam cross-linked porous sheet comprising the step of irradiating the electron beam and crosslinking.

At this time, the extruder is a single extruder or twin extruder, the extrusion temperature may be maintained at 50 to 250 ℃.

The solvent has a boiling point of 250 ° C. or lower, and is selected from the group consisting of paraffin solvent, olefin solvent, naphtha solvent, aromatic solvent, alcohol solvent, ester solvent, ketone solvent, and ether solvent. It may be any one or a mixture of two or more thereof.

The step (S5) may be to irradiate the accelerating electron beam with a voltage of 100 to 1,500 kV and an electron dose of 0.5 to 40 Mrad.

On the other hand, according to another aspect of the present invention, there is provided an electron beam crosslinked porous sheet prepared by the above-described method of the present invention.

At this time, the density of the electron beam cross-linked porous sheet is 0.20 to 0.80 g / cm ³ The porosity may be 10 to 80%, and the pore size may be 0.1 to 150 μm.

The crosslinking degree of the electron beam crosslinked porous sheet is 5 to 90%, and the compressive strength at the time of 25% compression is 0.1. To 3.0 kgf / cm ² , and the thickness may be 10 to 500 μm.

In addition, the absorption rate of the ball drop of the electron beam cross-linked porous sheet may be 20 to 60%.

In addition, the electron beam crosslinked porous sheet may be used for an impact absorber, a tape, or a slip sheet for electronic products.

The electron beam crosslinked porous sheet according to the present invention is excellent in performance as an impact absorber for electronic products by using a basic resin excellent in shock absorption.

Furthermore, by manufacturing the porous sheet through electron beam crosslinking, the crosslinking efficiency may be increased to improve physical strength and resilience.

The following drawings attached to this specification are illustrative of preferred embodiments of the present invention, and together with the detailed description of the invention to serve to further understand the technical spirit of the present invention, the present invention is a matter described in such drawings It should not be construed as limited to.
1 is a SEM photograph showing the side of the electron beam cross-linked porous sheet prepared according to an embodiment of the present invention.
Figure 2 is a SEM photograph showing the side of the electron beam cross-linked porous sheet prepared according to another embodiment of the present invention.

Hereinafter, the present invention will be described in detail. The terms or words used in this specification and claims are not to be construed as limiting in their usual or dictionary meanings, and the inventors may appropriately define the concept of terms in order to best explain their invention in the best way possible. It should be interpreted as meaning and concept corresponding to the technical idea of the present invention based on the principle that the present invention.

In addition, the configuration of the embodiments described herein is only one of the most preferred embodiments of the present invention and does not represent all of the technical idea of the present invention, various equivalents and modifications that may be substituted for them at the time of the present application It should be understood that there may be

The composition for a porous sheet according to the present invention, the base comprising any one or a mixture of two or more selected from the group consisting of thermoplastic polyolefin resin, thermoplastic polystyrene resin, thermoplastic rubber, silicone resin, fluorine resin, engineering plastics 10 to 80 parts by weight of the resin; 20 to 90 parts by weight of a plasticizer; And 0.1 to 5.0 parts by weight of the crosslinking aid.

As such, by satisfying the content of each composition, it is possible to generate an excellent shock absorbing effect, and can be shown to have a low compressive strength if made of a porous sheet later.

When the content of the base resin is less than 10 parts by weight, the pore size of the porous sheet is excessively increased, the compressive strength is significantly lowered, the impact absorbency may be lowered, and when it exceeds 80 parts by weight, the pore size of the porous sheet is excessively reduced In addition, the compressive strength may increase, which may lower the shock absorbency.

When the content of the plasticizer is less than 20 parts by weight, the melt viscosity is increased, which adversely affects the kneading property, and the extractability may be lowered. When the content of the plasticizer is greater than 90 parts by weight, the melt viscosity may be lowered, making the sheet difficult to form. Can not do it.

When the content of the crosslinking aid is less than 0.1 part by weight, the crosslinking degree is low, the compressive strength may be significantly lowered. When it exceeds 5 parts by weight, the crosslinking degree is excessively high, the compressive strength may be increased, and the impact absorption may be deteriorated. .

Here, the thermoplastic polyolefin-based resin includes a thermoplastic modified polyolefin elastomer-based resin, the engineering plastics, polycarbonate-based resin, terephthalate-based resin, polyamide-based resin and the like, wherein the terephthalate-based resin And polyethylene terephthalate resin, polypropylene terephthalate resin, and polybutylene terephthalate resin.

The plasticizer may be liquid at room temperature, and may be any one or a mixture of two or more selected from the group consisting of a paraffinic plasticizer, an olefin plasticizer, a naphtha plasticizer, and an aromatic plasticizer.

In addition, the crosslinking aid improves the crosslinking efficiency during electron beam crosslinking, and serves to control the crosslinking degree for increasing physical properties such as tensile strength and tear strength of the porous sheet.

The crosslinking aid that may be used herein may be any one selected from the group consisting of vinyl monomers, acrylate compounds, methacrylate compounds, and epoxy compounds, or a mixture of two or more thereof.

More specifically, a crosslinking aid having a bifunctional group, a trifunctional group or a tetrafunctional group or more may be used. Among the bifunctional crosslinking aids, as the diacrylate type, butanediol diacrylate (BDDA), 1,6-hexanediol diol Acrylate (HDDA), tetraethylene glycol diacrylate (TTEGDA), and the like, and dimethacrylate-based hexanediol dimethacrylate (HDDMA). And among trifunctional crosslinking adjuvant, trimethylol propane triacrylate (TMPTA), pentaerythritol triacrylate (PETA), glyceryl hydroxylated triacrylate (GPTA), etc. are mentioned as a triacrylate type, Examples of the methacrylate type include trimethylolpropane trimethacrylate (TMPTMA). And as a crosslinking adjuvant which has a tetrafunctional group or more, there exist a tetraacrylate type | system | group, a penta acrylate system, etc.

Hereinafter, a method of manufacturing the electron beam crosslinked porous sheet according to the present invention will be described.

First, 10 to 80 parts by weight of a base resin including any one or a mixture of two or more selected from the group consisting of a thermoplastic polyolefin resin, a thermoplastic polystyrene resin, a thermoplastic rubber, a silicone resin, a fluorine resin, and an engineering plastic; 20 to 90 parts by weight of a plasticizer; And 0.1 to 5.0 parts by weight of the crosslinking aid is kneaded to prepare a raw material composition (step S1). This mixing process can be carried out in a wet, high pressure reaction kneader.

Subsequently, the raw material composition is introduced into an extruder and extruded (step S2).

Here, the step (S2) is a process of extruding the raw material composition prepared in the step (S1), a twin extruder (single extruder) or a twin extruder (single feed into the side feeding method using a separate screw) extruder), and the raw material composition is kneaded and extruded as an extruder having a screw having a temperature of 50 to 250 ° C and a cylinder having a temperature of 50 to 250 ° C. There is an effect of uniformly mixing the whole composition contained in the raw material composition prepared in step (S1). In addition, if the temperature of the cylinder and die is kept below 50 ° C., the resin mixture may be insignificantly melted, and the kneading property of the mixture may be reduced. Therefore, it is preferable to maintain the temperature of the cylinder and die of an extruder at 50-250 degreeC.

Subsequently, the extrudate is cast (step S3). At this time, the extrudate is cast to produce a sheet-like product, and the width and thickness of the product can be variously adjusted according to the judgment of those skilled in the art.

Subsequently, the resultant of the step (S3) is immersed in a solvent to form pores by extracting the plasticizer (step S4).

Here, the solvent may be used a variety of polar solvents and non-polar solvents depending on the type of the plasticizer.

However, after the plasticizer has been extracted, the remaining solvent must be removed, and therefore the boiling point of the solvent should not be too high, but is preferably 250 ° C. or lower.

As such a solvent, various solvents such as a paraffin solvent, an olefin solvent, a naphtha solvent, an aromatic solvent, an alcohol solvent, an ester solvent, a ketone solvent and an ether solvent can be used.

Subsequently, after drying the resultant of the step (S4), a porous sheet is prepared by irradiating an electron beam and crosslinking (step S5).

Here, the crosslinking method through the electron beam irradiation is more environmentally friendly than the conventional chemical crosslinking method, it is excellent in economic efficiency and can further secure process stability.

At this time, it is preferable to irradiate the accelerated electron beam with a voltage of 100 to 1,500 kV and an electron beam dose of 0.5 to 40 Mrad, and the adjustment of the crosslinking voltage is appropriate depending on the thickness of the resultant pores obtained through the step (S4). The amount of crosslinked electron beam may be appropriately adjusted according to the density and physical properties of the base resin.

When the voltage is less than 100 kV, crosslinking is not performed to a sufficient depth with respect to the resultant product in which the pores are formed. When the voltage is more than 1,500 kV, due to energy waste and radiation leakage due to excessive irradiation amount, economic efficiency and safety are lowered.

When the electron beam amount is less than 0.5 Mrad, sufficient crosslinking is not performed with respect to the resultant product in which the pores are formed, and product formation is not performed. When the electron beam amount exceeds 40 Mrad, the crosslinking reaction is in a supersaturated state due to excessive crosslinking electron beam dose. This not only leads to poor product formation, but also economics.

In the crosslinking method using electron beam irradiation proceeding under the conditions as described above, the accelerated electron beam is irradiated to the object of the crosslinking to remove hydrogen contained in the object, thereby generating radicals in the resin mixture. The radicals thus produced have high reactivity and are characterized by crosslinking reaction in the resin mixture.

At this time, the cross-linking degree of the prepared porous sheet may be 5 to 90%, more preferably 10 to 70%, by satisfying the cross-linking degree, the effect of improving the compression set is generated.

In general, the crosslinking of the polymer improves mechanical properties, in particular, tensile strength and compressive strength. When the porous sheet is manufactured by extracting the plasticizer as in the present application, the strength of the resin forming the porous structure is lowered. Therefore, when an external impact is applied, the tensile strength is lowered, the porous structure is destroyed, or the restorability, that is, the compressive permanent strain is lowered, thereby reducing the repeated shock absorption.

However, as described herein, when the degree of crosslinking of the porous sheet satisfies the range of 5 to 90%, the compressive permanent strain may be improved to prevent the impact absorbency from being lowered. If the crosslinking degree is in the range of less than 5%, due to excessive decrease in the crosslinking degree, the porous structure may be destroyed, and thus shock absorbency and recoverability (compressive permanent strain) may be lowered, which is not preferable, and the crosslinking degree exceeds 90%. In this case, the rebound elasticity is improved by increasing the compressive strength and the shock absorbency is lowered, which is not preferable.

Meanwhile, the electron beam crosslinked porous sheet has a thickness of 10 to 500 µm, a density of 0.2 to 0.8 g / cm ³ , a pore size of 0.1 to 150 µm, a degree of crosslinking of 5 to 90%, and 25% compression. The compressive strength of may be 0.1 to 3.0 kgf / cm ² , with respect to the shock absorption rate, the point shock absorption rate may be 20 to 60%.

When the above properties are satisfied, when used as a shock absorber for electronic products such as mobile phones, it can exhibit excellent performance.

Here, the point shock absorption rate measurement method will be described in detail.

First, a stack of a steel plate and a SUS plate is placed on the impact amount measuring sensor, and a 55 g steel ball is dropped directly on the SUS plate at a height of 50 cm to measure the impact force F1 through the sensor.

Subsequently, a porous sheet specimen (67 mm * 67 mm) is attached between the steel plate and the SUS plate, and the steel sphere is dropped directly on the SUS plate at a height of 50 cm to measure the impact force F2 through the sensor.

Using the impact forces (F1, F2) it is possible to calculate the point shock absorption rate through the following equation.

Point Shock Absorption = (F1-F2) / F1 × 100 (%)

Hereinafter, the present invention will be described in detail with reference to Examples. However, embodiments according to the present invention can be modified in many different forms, the scope of the present invention should not be construed as limited to the embodiments described below. The embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

Example 1

A raw material composition was prepared by kneading 20 parts by weight of thermoplastic polyethylene elastomer resin as a base resin, 79 parts by weight of liquid paraffin oil as a plasticizer, and 1 part by weight of trimethylolpropane trimethacrylate as a crosslinking aid.

Subsequently, the raw material composition was introduced into a twin extruder maintained at 120 ° C., extruded, and then the extrusion product was cast, and the resultant was immersed in an n-hexane solvent to extract the plasticizer and dried. Thereafter, a 500 kV voltage and 5.0 Mrad electron beam were irradiated to crosslink the product from which the plasticizer was extracted to prepare a porous sheet.

2. Example 2

A raw material composition was prepared by kneading 20 parts by weight of thermoplastic polystyrene resin as a base resin, 79 parts by weight of liquid paraffin oil as a plasticizer, and 1 part by weight of trimethylolpropane trimethacrylate as a crosslinking aid.

Subsequently, the raw material composition was introduced into a twin extruder maintained at 120 ° C., extruded, and then the extrusion product was cast, and the resultant was immersed in an n-hexane solvent to extract the plasticizer and dried. Thereafter, a 500 kV voltage and 5.0 Mrad electron beam were irradiated to crosslink the product from which the plasticizer was extracted to prepare a porous sheet.

3. Comparative Example 1

A raw material composition was prepared by kneading 20 parts by weight of thermoplastic polystyrene resin as a base resin and 80 parts by weight of liquid paraffin oil as a plasticizer.

Subsequently, the raw material composition was introduced into a twin extruder maintained at 120 ° C., extruded, and then the extrusion product was cast, and the resultant was immersed in an n-hexane solvent to extract the plasticizer and dried. Thereafter, a 500 kV voltage and 5.0 Mrad electron beam were irradiated to crosslink the product from which the plasticizer was extracted to prepare a porous sheet.

4. Comparative Example 2

A raw material composition was prepared by kneading 85 parts by weight of a thermoplastic polystyrene resin as a basic resin, 14 parts by weight of liquid paraffin oil as a plasticizer, and 1 part by weight of trimethylolpropane trimethacrylate as a crosslinking aid.

Subsequently, the raw material composition was introduced into a twin extruder maintained at 120 ° C., extruded, and then the extrusion product was cast, and the resultant was immersed in an n-hexane solvent to extract the plasticizer and dried. Thereafter, a 500 kV voltage and 5.0 Mrad electron beam were irradiated to crosslink the product from which the plasticizer was extracted to prepare a porous sheet.

5. Comparative Example 3

A raw material composition was prepared by kneading 8 parts by weight of a thermoplastic polystyrene resin as a basic resin, 91 parts by weight of liquid paraffin oil as a plasticizer, and 1 part by weight of trimethylolpropane trimethacrylate as a crosslinking aid.

Subsequently, the raw material composition was introduced into a twin extruder maintained at 120 ° C., extruded, and then the extrusion product was cast, and the resultant was immersed in an n-hexane solvent to extract the plasticizer and dried. Thereafter, a 500 kV voltage and 5.0 Mrad electron beam were irradiated to crosslink the product from which the plasticizer was extracted to prepare a porous sheet.

6. Comparative Example 4

A raw material composition was prepared by kneading 20 parts by weight of thermoplastic polystyrene resin as a base resin, 79 parts by weight of liquid paraffin oil as a plasticizer, and 1 part by weight of trimethylolpropane trimethacrylate as a crosslinking aid.

Subsequently, the raw material composition was introduced into a twin extruder maintained at 120 ° C., extruded, and then the extrusion product was cast, and the resultant was immersed in an n-hexane solvent to extract the plasticizer and dried. Then, a porous sheet was prepared by crosslinking the resultant from which the plasticizer was extracted by irradiating a voltage of 500 kV and an electron beam of 50.0 Mrad.

The compositions of the porous sheets prepared by the above Examples and Comparative Examples are summarized in Table 1 below, and their physical properties are shown in Table 2 below.

Formulation Kinds Example 1 Example 2 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Basic resin Polyolefin elastomer 20 0 0 0 0 0 Polystyrene resin 0 20 20 85 8 20 Plasticizer Paraffin oil 79 79 80 14 91 79 Crosslinking aid Trimethylolpropane trimethacrylate One One 0 One One One Parts by weight 100 100 100 100 100 100 Crosslinking irradiation dose (500 kV) 5 5 5 5 5 50

characteristic Example 1 Example 2 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Thickness (㎛) 130 130 130 130 120 140 Density (g / cm ³ ) 0.23 0.22 0.22 0.43 0.07 0.21 Pore size (㎛) 5 20 35 - 250 25 Degree of crosslinking (%) 20 35 0.5 45 15 75 Compressive strength (kgf / cm ² ) 0.4 0.2 0.05 5.0 0.08 4.0 Point Shock Absorbency (%) 40 55 15 5 17 20

On the other hand, the embodiments of the present invention disclosed herein are not intended to limit the scope of the present invention only presented a specific example to facilitate understanding. It will be apparent to those skilled in the art that other modifications based on the technical idea of the present invention can be carried out in addition to the embodiments disclosed herein.

Claims (15)

10 to 80 parts by weight of a base resin including any one or a mixture of two or more thereof selected from the group consisting of a thermoplastic polyolefin resin, a thermoplastic polystyrene resin, a thermoplastic rubber, a silicone resin, a fluorine resin, and an engineering plastic;
20 to 90 parts by weight of a plasticizer; And
Composition for a porous sheet containing 0.1 to 5.0 parts by weight of the crosslinking aid. The method of claim 1,
The plasticizer is a liquid at room temperature,
A composition for a porous sheet, which is any one selected from the group consisting of paraffin-based plasticizers, olefin-based plasticizers, naphtha-based plasticizers and aromatic plasticizers, or a mixture of two or more thereof. The method of claim 1,
The crosslinking aid is a composition for a porous sheet, characterized in that any one or a mixture of two or more selected from the group consisting of vinyl monomers, acrylate compounds, methacrylate compounds and epoxy compounds. (S1) 10 to 80 parts by weight of a base resin including any one or a mixture of two or more selected from the group consisting of a thermoplastic polyolefin resin, a thermoplastic polystyrene resin, a thermoplastic rubber, a silicone resin, a fluorine resin, and an engineering plastic; 20 to 90 parts by weight of a plasticizer; And kneading 0.1 to 5.0 parts by weight of the crosslinking aid to prepare a raw material composition.
(S2) inserting the raw material composition into an extruder to extrude;
(S3) casting the result of the step (S2);
(S4) forming pores by dipping the resultant of the step (S3) in a solvent to extract the plasticizer; And
(S5) After drying the product of the step (S4), the method of producing an electron beam crosslinked porous sheet comprising the step of irradiating an electron beam and crosslinking. The method of claim 4, wherein
The extruder is a single extruder or twin extruder, the extrusion temperature is a method for producing an electron beam cross-linked porous sheet, characterized in that maintained at 50 to 250 ℃. The method of claim 4, wherein
The solvent has a boiling point of 250 ° C. or less,
Paraffin solvent, olefin solvent, naphtha solvent, aromatic solvent, alcohol solvent, ester solvent, ketone solvent and ether solvent selected from the group consisting of any one or a mixture of two or more of them Method for producing an electron beam crosslinked porous sheet. The method of claim 4, wherein
In the step (S5), the electron beam cross-linked porous sheet is characterized in that for irradiating the accelerated electron beam with a voltage of 100 to 1500 kV and an electron beam amount of 0.5 to 40 Mrad. An electron beam crosslinked porous sheet produced by the manufacturing method according to any one of claims 4 to 7. The method of claim 8,
The electron beam crosslinked porous sheet has a density of 0.20 to 0.80 g / cm ³ Electron beam crosslinked porous sheet, characterized in that the porosity is 10 to 80%. The method of claim 8,
Electron beam crosslinked porous sheet, characterized in that the pore size of the electron beam crosslinked porous sheet is 0.1 to 150 ㎛. The method of claim 8,
Electron beam crosslinked porous sheet, characterized in that the crosslinking degree of the electron beam crosslinked porous sheet is 5 to 90%. The method of claim 8,
Compressive strength at 25% compression of the electron beam crosslinked porous sheet is 0.1 To 3.0 kgf / cm ² electron beam crosslinked porous sheet. The method of claim 8,
The electron beam crosslinked porous sheet has a thickness of 10 to 500 μm. The method of claim 8,
Electron beam cross-linked porous sheet, characterized in that the ball drop test absorption rate of the electron beam cross-linked porous sheet is 20 to 60%. The method of claim 8,
The electron beam crosslinked porous sheet is an electron beam crosslinked porous sheet, characterized in that it is used in impact absorbers, tapes or slippers for electronic products.

Images