HK1229291B - Embossing tool and method for preparation thereof - Google Patents

Description

Embossing tool and preparation method thereof

Technical Field

The present invention relates to an embossing tool, an assembly for producing such an embossing tool, and a method for producing such an embossing tool.

Background Art

Embossing tools are usually made of nickel, copper, alloys or other types of composite materials. Nickel is the most widely used material for embossing machine manufacturing.

There are a number of problems associated with currently available embossing tools, particularly incomplete release of material that solidifies after embossing or heat embossed material from the embossing tool.

There are many methods for modifying the surface of embossing tools to reduce the adhesion between the surface of the embossing tool and the cured or heat-embossed material. These methods may include silane coating, silicone resin coating, PTFE (polytetrafluoroethylene) coating, or nickel-PTFE composite plating. Unfortunately, none of them can produce satisfactory results.

Silicone resin and PTFE can be applied to the surface of the embossing tool by wet coating. However, after drying and curing, the thickness uniformity of the coating on the microstructure surface is poor, which may change the shape of the microstructure obtained on the embossing tool.

When the microstructures on the surface of the embossing tool have a high aspect ratio, PTFE coatings deposited by physical vapor deposition (PVD) or chemical vapor deposition (CVD) have shown poor dispersion and uneven coverage. In addition, the poor durability and mechanical strength of the PTFE coating are additional concerns, especially if the embossing tool is to be widely used in mass production.

Nickel-PTFE composite coatings can be applied to the surface of embossing tools by electroplating or electroless plating. However, the minimum coating thickness is typically several micrometers. Therefore, if the embossing tool has small-scale microstructures on its surface, especially narrow grooves, such coatings can drastically change the microstructure's profile and aspect ratio, making embossing much more difficult.

Published U.S. Patent Application No. 2016/0059442 and Taiwan Patent Application No. 104129779 describe an embossing tool having a microstructure on its surface, wherein the microstructured surface is coated with a precious metal or precious metal alloy. The tool is produced by first forming the microstructure using conventional photolithographic techniques and then coating these microstructures with a precious metal or precious metal alloy. The present invention relates to variations of this method for forming the embossing tool and to structures produced during this method.

Summary of the Invention

Therefore, the present invention provides a method for preparing an embossing tool, the method comprising:

a) forming a mold having a non-planar mold surface defining the outer shape of the embossing tool;

b) coating a layer of gold or its alloy on the surface of the mold;

c) plating a base metal on the gold or alloy layer to form a base metal layer having a substantially flat surface away from the mold surface; and

d) removing the mold from the gold or alloy thereof layer and the base metal layer to form an embossing tool having a three-dimensional structure on one side and a substantially flat surface on the other side.

In one embodiment, the embossing tool prepared in step d) is then wrapped around a roller.The mold can be formed by coating a photoresist material on a substrate, exposing the photoresist material to radiation, and removing either the exposed or unexposed areas of the photoresist.

The present invention also provides an assembly for preparing an embossing tool, the assembly comprising:

molds having non-planar mold surfaces;

a layer of gold or its alloy disposed on the mold surface and conforming to the mold surface; and

A base metal layer on the opposite side of the gold or alloy layer away from the mold surface, the base metal layer having a three-dimensional structure in contact with the gold or alloy layer and a substantially flat surface on the side away from the gold or alloy layer.

BRIEF DESCRIPTION OF THE DRAWINGS

1A and 1B illustrate a general embossing process.

FIG. 2 shows a prior art method for forming a microstructure on the surface of an embossing tool.

FIG3 is a cross-section through a prior art embossing tool as described in the aforementioned US 2016/0059442, having a three-dimensional microstructure and a precious metal (eg, gold) plated on its surface.

FIG4 illustrates the method for forming an embossing tool described in the aforementioned US 2016/0059442.

FIG. 5 illustrates the method of the present invention for forming an embossing tool.

FIG. 6A is a photograph showing the surface of an object produced by an embossing process using a conventional embossing tool.

FIG. 6B is a photograph showing the surface of an object produced by an embossing process using the embossing tool of the present invention.

DETAILED DESCRIPTION

Figures 1A and 1B illustrate an embossing process using an embossing tool (11) having a three-dimensional microstructure (circled) on its surface. As shown in Figure 1B, after the embossing tool (11) is applied to a curable embossing composition or heat-embossable material (12), and the embossing composition is cured (e.g., by radiation) or the heat-embossable material is embossed by heat and pressure, the cured or heat-embossed material is released from the embossing tool (see Figure 1B). However, when using conventional embossing tools, the cured or heat-embossed material is sometimes not fully released from the tool due to undesirably strong adhesion between the cured or heat-embossed material and the surface of the embossing tool. In such cases, some of the cured or heat-embossed material may be transferred to or adhere to the surface of the embossing tool, leaving an uneven surface on the object formed by the process.

This problem is even more pronounced if the object is formed on a support layer, such as a transparent conductive layer or polymer layer. If the adhesion between the cured or heat-embossed material and the support layer is weaker than the adhesion between the cured or heat-embossed material and the surface of the embossing tool, then the release of the cured or heat-embossed material from the embossing tool may cause the object to separate from the support layer.

In some cases, the object may be formed from stacked layers, in which case the release of the cured or heat-embossed material from the embossing tool may result in a split between any two adjacent layers if the adhesion between the two layers is weaker than the adhesion between the cured or heat-embossed material and the surface of the embossing tool.

These issues are particularly concerning when the cured embossing composition or heat-embossed material does not adhere well to certain support layers. For example, if the support layer is a polymer layer, then if one of the layers is hydrophilic and the other is hydrophobic, the adhesion between the polymer layer and the cured or heat-embossed embossing composition is weak. Therefore, it is preferred that both the embossing composition and the support layer be hydrophobic or both be hydrophilic.

As an example, suitable hydrophobic compositions for forming the embossing layer or the support layer may include thermoplastic materials, thermosetting materials, or precursors thereof. Examples of thermoplastic materials or thermosetting material precursors may be multifunctional acrylates or methacrylates, multifunctional vinyl ethers, multifunctional epoxides, and oligomers or polymers thereof.

Suitable hydrophilic compositions for forming the embossing layer or the support layer may include polar oligomers or polymers. As described in U.S. Pat. No. 7,880,958, such polar oligomers or polymers may be selected from the group consisting of oligomers or polymers having at least one group such as a nitro group (—NO ₂ ), a hydroxyl group (—OH), a carboxyl group (—COO), an alkoxy group (—OR, wherein R is an alkyl group), a halogen group (e.g., fluorine, chlorine, bromine, or iodine), a cyano group (—CN), and a sulfonic acid group (—SO ₃ ). The glass transition temperature of the polar polymer material is preferably below about 100° C., and more preferably below about 60° C. Specific examples of suitable polar oligomeric or polymeric materials can include, but are not limited to, polyvinyl alcohol, polyacrylic acid, poly(2-hydroxyethyl methacrylate), polyhydroxy-functionalized polyester acrylates (such as BOE 1025, Bomar Specialties Co., Winsted, CT), or alkoxylated acrylates, such as ethoxylated nonylphenol acrylate (e.g., SR504, Sartomer Company), ethoxylated trimethylolpropane triacrylate (e.g., SR9035, Sartomer Company), or ethoxylated pentaerythritol tetraacrylate (e.g., SR494 from Sartomer Company).

Method 1:

FIG. 2 shows a prior art method for forming a microstructure on the surface of an embossing tool.

As used herein, the term "embossing tool" may refer to an embossing sleeve, an embossing drum, or other forms of embossing tool. Although only the preparation of an embossing sleeve is shown in FIG2 , the method can also be used to prepare an embossing drum. The term "embossing" drum or sleeve refers to a drum or sleeve having a three-dimensional microstructure on its outer surface. The term "embossing drum" is used to distinguish it from a plain drum that does not have a three-dimensional microstructure on its outer surface.

The embossing cylinder can be used directly as the embossing tool. When an embossing sleeve is used for embossing, it is usually mounted on a smooth cylinder to allow the embossing sleeve to rotate.

The embossing roller or sleeve (21) is typically formed of a conductive material such as a metal (e.g., aluminum, copper, zinc, nickel, chromium, titanium, or cobalt), an alloy derived from any of the above metals, or stainless steel. A variety of materials may be used to form the roller or sleeve. For example, the center of the roller or sleeve may be formed of stainless steel, with a nickel layer sandwiched between the stainless steel and an outermost layer, which may be a copper layer.

Alternatively, the embossing cylinder or sleeve (21) may be formed from a non-conductive material having a conductive coating or a conductive seed layer on its outer surface.

Before coating the photosensitive material (22) on the outer surface of the drum or sleeve (21), as shown in step B of FIG. 2, precision grinding and polishing may be used to ensure the smoothness of the outer surface of the drum or sleeve.

In step B, a photosensitive material (22), such as a photoresist, is applied to the outer surface of the roller or sleeve (21). The photosensitive material can be positive tone, negative tone, or dual tone. The photosensitive material can also be a chemically amplified photoresist. Coating can be performed using dip coating, spray coating, or ring coating. After drying and/or baking, the photosensitive material is exposed to a radiation source, as shown in step C.

Alternatively, the photosensitive material (22) may be a dry film photoresist (which is commonly available commercially) which is laminated to the outer surface of the cylinder or sleeve (21). When a dry film is used, it may also be exposed to a radiation source as described below.

In step C, a suitable light source (23), such as IR, UV, electron beam or laser, is used to expose the photosensitive material coated on the roller or sleeve (21) or the dry film photoresist (22) laminated on the roller or sleeve (21). The light source can be continuous light or pulsed light. A photomask (24) is optionally used to define the three-dimensional microstructure to be formed. Depending on the microstructure, the exposure can be step-by-step, continuous or a combination thereof.

After exposure and before development, the photosensitive material (22) may undergo post-exposure treatment, such as baking. Depending on the type of photosensitive material, the exposed areas or the unexposed areas will be removed by using a developer. After development and before deposition (e.g., electroplating, chemical plating, physical vapor deposition, chemical vapor deposition, or sputtering deposition), the roller or sleeve having the patterned photosensitive material (25) on its outer surface (as shown in step D) may undergo baking or full exposure. The thickness of the patterned photosensitive material is preferably greater than the depth or height of the three-dimensional microstructure to be formed.

A metal or alloy (e.g., nickel, cobalt, chromium, copper, zinc, or alloys derived from any of the foregoing metals) may be electroplated and/or electrolessly plated onto the drum or sleeve. The plating material (26) is deposited on the outer surface of the drum or sleeve in areas not covered by the patterned photosensitive material. The deposited thickness is preferably less than the thickness of the photosensitive material, as shown in step E. By adjusting the plating conditions, such as the distance between the anode and cathode (i.e., the drum or sleeve) (if electroplating is used), the rotation speed of the drum or sleeve, and/or the circulation of the plating solution, the thickness variation of the deposit over the entire drum or sleeve area can be controlled to be less than 1%.

Alternatively, where electroplating is used to deposit the plating material (26), the thickness variation of the deposit across the surface of the roller or sleeve can be controlled by inserting a non-conductive thickness uniformer between the cathode (i.e., roller or sleeve) and the anode, as described in U.S. Patent No. 8,114,262.

After plating, the patterned photosensitive material (25) can be stripped by a stripper (eg, an organic solvent or aqueous solution). Precision polishing can optionally be employed to ensure acceptable thickness variation and roughness of the deposit (26) across the drum or sleeve.

Step F of FIG. 2 shows a cross section through an embossing cylinder or sleeve having a three-dimensional pattern of microstructures formed thereon.

As described in the aforementioned US2016/0059442, it has been found that if the surface of the embossing tool is coated with a noble metal or an alloy thereof, the embossing tool can have improved release properties. In other words, as a post-processing step after forming a three-dimensional microstructure on the surface of the embossing tool, the noble metal or an alloy thereof (31) can be coated on the entire surface of the embossing tool, as shown in FIG3.

The term "noble metal" may include gold, silver, platinum, palladium and other less common metals such as ruthenium, rhodium, osmium or iridium. Of these noble metals, the inventors have found that gold and its alloys are most effective in reducing the adhesion between the cured or hot-embossed material and the surface of the embossing tool. This advantage is particularly evident when the cured or hot-embossed material has one or more of the following components: polyacrylates, polymethyl methacrylate (PMMA), polyethyl methacrylate (PEMA), polycarbonate (PC), polyvinyl chloride (PVC), polystyrene (PS), polyesters, polyamides, polyurethanes, polyolefins, polyvinyl butyral and copolymers thereof. Of these cured or hot-embossed materials, polymers based on acrylates or methacrylates are particularly preferred.

In the present invention, alloys of one or more noble metals and non-noble metals may also be used. Suitable non-noble metals in the alloys may include, but are not limited to, copper, tin, cobalt, nickel, iron, indium, zinc, or molybdenum. More than one noble metal and/or more than one non-noble metal may also be present in the alloy. The total weight percentage of non-noble metals in the alloy may range from 0.001% to 50%, preferably from 0.001% to 10%.

The coating of precious metals or alloys can be achieved by electroplating, chemical deposition, sputter coating, or vapor deposition. In one embodiment, a cyanide-based neutral gold, acidic hard gold, or gold strike plating electrolyte can be used at a temperature of 30-70°C and a pH range of 3-8. Platinum and palladium can be plated using an acidic chloride electrolyte at a temperature of 40-70°C and a pH range of 0.1-3. Alkaline electrolytes for some precious metals or their alloys are commercially available and can also be used in the present invention.

The noble metal or its alloy on the surface preferably has a submicron thickness so that it does not cause any significant changes to the profile of the microstructure. The thickness of the noble metal or its alloy may be in the range of 0.001-10 microns, preferably in the range of 0.001-3 microns.

Method 2:

Alternatively, as described in the aforementioned US 2016/0059442, a three-dimensional microstructure may be formed on a flat substrate, as shown in FIG. 4 .

In step A of FIG4 , a photosensitive material (42) is applied to a substrate layer (41) (e.g., a glass substrate). As described above, the photosensitive material can be positive, negative, or dual. The photosensitive material can also be a chemically amplified photoresist. Coating can be performed using dip coating, spray coating, slot die coating, or spin coating. After drying and/or baking, the photosensitive material is exposed to a suitable light source (not shown) through a photomask (not shown).

Alternatively, the photosensitive material (42) can be a dry film photoresist (which is commonly available commercially) laminated onto the substrate (41). The dry film can also be exposed to a light source as described above.

In step B, after exposure, the exposed or unexposed areas of the photosensitive material are removed by using a developer, depending on the type of photosensitive material. After development, the substrate layer (41) with the remaining photosensitive material (42) can be baked or fully exposed before step C. The thickness of the remaining photosensitive material should be the same as the depth or height of the three-dimensional microstructure to be formed.

In step C, a conductive seed layer (43) is applied over the remaining photosensitive material (42) and the areas on the substrate (41) not occupied by the photosensitive material. The conductive seed layer is typically formed of silver.

In step D, a metal or alloy (44) (e.g., nickel, cobalt, chromium, copper, zinc, or alloys derived from any of the above metals) is electroplated and/or electrolessly plated on the surface covered by the conductive seed layer, and the plating process is performed until a sufficient plated material thickness (h) is formed on the patterned photosensitive material. The thickness (h) in FIG4 is preferably 25-5000 microns, and more preferably 25-1000 microns.

After plating, the plated material (44) is separated from the stripped substrate layer (41). The photosensitive material (42) is removed along with the conductive seed layer (43). The photosensitive material can be removed using a stripper (e.g., an organic solvent or aqueous solution). The conductive seed layer (43) can be removed using an acidic solution (e.g., a sulfur/nitrogen mixture) or a commercially available chemical stripper, leaving only a metal sheet (44) with a three-dimensional structure on one side and a flat surface on the other.

A precision polish can be applied to the metal sheet (44), after which the smooth shim can be used directly for embossing. Alternatively, it can be mounted (eg wrapped) on a roller having a three-dimensional microstructure on its outer surface to form an embossing tool.

As mentioned above, the precious metal or its alloy is finally coated on the entire surface of the embossing tool.As mentioned above, gold or its alloy is preferred over other precious metals and alloys.

Method 3:

FIG5 shows the method of the present invention. The method is similar to the method of FIG4 , but simplified. Steps A and B of FIG5 are the same as the corresponding steps of FIG4 . However, in step C of FIG5 , a layer of gold or its alloy (53) is applied instead of a conductive seed layer such as silver.

Therefore, in step E of Figure 5, after separating the plated material (54) from the substrate (51), only the photosensitive material (52) needs to be removed, and the gold or alloy coating (53) remains together with the metal sheet (54) having a three-dimensional structure on one side and a flat surface on the other side.

The metal sheet can be used directly for embossing. Alternatively, it can be mounted on a roller. In the method of the present invention, there is no need for a separate coating step to form a gold or alloy layer on the surface of the embossing tool.

The embossing tool of the present invention is suitable for use in a micro-embossing process as described in U.S. Patent No. 6,930,818. The micro-embossing process produces cup-shaped microcells separated by partitions, such as MICROCUPS (registered trademark). These microcells can be filled with an electrophoretic fluid containing charged particles dispersed in a solvent or solvent mixture. The filled microcells form an electrophoretic display film. When sandwiched between electrode layers, the electrophoretic display film forms an electrophoretic device.

Example

Example 1

In this example, two embossing tools (ie, male dies) were prepared. These dies were formed from nickel according to one of the methods described above.

The surface of one of these nickel molds was not treated. The other nickel mold formed was further electroplated with a cyanide-based gold plating electrolyte at a temperature of 50° C. and pH 5 to obtain a gold coating having a thickness of 0.5 μm on the surface thereof.

To test the two embossing dies, a water-based polymer layer fluid and an embossing composition were prepared. The polymer layer fluid was prepared according to U.S. Patent No. 7,880,958 and contained polyvinyl alcohol as a primary component. The embossing composition was prepared according to U.S. Patent No. 7,470,386 and contained a multifunctional acrylate as a primary component.

The polymer fluid was first coated onto a PET (polyethylene terephthalate) substrate using a Meyer drawdown bar No. 3. The dried polymer layer had a thickness of 0.5 microns.

The embossing composition was diluted with methyl ethyl ketone (MEK) and then coated on the polymer layer side of the PET substrate with a target dry thickness of 25 microns.

Using both embossing dies, the coatings were dried and embossed separately with UV exposure (0.068 J/ ^cm2 , Fusion UV, D lamp) through the back side of the PET substrate at 160°F (71°C) under 50 psi (350 kPa) pressure.

Fig. 6A is a micrograph of the surface of a film prepared by using a nickel embossing die. It can be seen that due to the strong adhesion between the solidified material and the nickel metal, some of the solidified material on the resulting film has been transferred to the nickel die or stuck to the nickel die, leaving an uneven surface on the resulting film.

When a gold-plated nickel mold was used, the solidified embossed material separated completely from the gold metal surface, leaving a smooth surface on the resulting film, as shown in Figure 6B. This is due to the fact that the gold-plated surface reduced the adhesion between the mold surface and the solidified material, making the mold easier to release from the solidified material.

Example 2

In this example, several embossing tools (i.e., male molds) were prepared. These molds were formed from nickel according to one of the methods described above. One of the nickel molds was further electroplated with 0.5 microns of gold using the same electrolyte bath as used in Example 1.

Three of the resulting nickel molds were further subjected to silane surface treatment. For the silane treatment, polydimethylsiloxane (Gelest, Inc.) was added to a mixture of 95% n-propanol and 5% DI water, which had been previously adjusted to pH 4.5 with acetic acid. Three polydimethylsiloxane solutions were prepared at concentrations of 0.25%, 1%, and 2% by weight. The nickel molds were immersed in each of the silane solutions for 10 minutes and then baked at 100°C overnight to obtain a silane coating on the surface of the microstructures.

The embossing test materials and conditions were the same as those used in Example 1. When using a gold-plated nickel mold, all of the cured embossed material completely separated from the gold metal surface. However, regardless of the polydimethylsiloxane concentration in the processing solution, more than about 50% of the area of the cured embossed material on the resulting film had been transferred to or adhered to the silane-treated nickel mold surface.

This example demonstrates that the cured material releases more readily from a gold-plated surface than from a silane-treated surface.

Claims (15)

1. A method for preparing embossing tools, the method comprising: a) A mold having a non-planar mold surface that defines the shape of the embossing tool; b) Deposit a layer of gold or its alloy on the surface of the mold, wherein the gold or its alloy has a thickness of 0.5-10 micrometers; c) Depositing a base metal onto the gold or its alloy layer to form a base metal layer having a substantially flat surface away from the mold surface; and d) Remove the mold from the gold or its alloy layer and the base metal layer to form an embossing tool with a three-dimensional structure on one side and a substantially flat surface on the other side. 2. The method according to claim 1, wherein the embossing tool prepared in step d) is then wrapped around a roller. 3. The method of claim 1, wherein the mold is formed by: coating a photoresist material on a substrate, exposing the photoresist material to radiation, and removing the exposed or unexposed areas of the photoresist. 4. The method of claim 1, wherein the gold alloy comprises gold and one or more of the following: copper, tin, cobalt, nickel, iron, indium, zinc, and molybdenum. 5. The method according to claim 4, wherein the total weight of non-precious metals in the alloy is in the range of 0.001-50%. 6. The method according to claim 4, wherein the total weight of non-precious metals in the alloy is in the range of 0.001-10%. 7. The method of claim 1, wherein the gold or its alloy layer has a thickness of 0.5-3 micrometers. 8. The method of claim 1, wherein the base metal comprises one or more of nickel, cobalt, chromium, copper and zinc. 9. The method of claim 1, wherein the minimum thickness of the base metal layer is in the range of 25-5000 micrometers. 10. The method of claim 1, wherein the minimum thickness of the base metal layer is in the range of 25-1000 micrometers. 11. A component for preparing an embossing tool, the component comprising: A mold with a non-planar mold surface; A gold or alloy layer disposed on and adhered to the surface of the mold, wherein the gold or alloy layer has a thickness of 0.5-10 micrometers; and A base metal layer on the opposite side of the gold or alloy layer away from the mold surface, the base metal layer having a three-dimensional structure in contact with the gold or alloy layer and a substantially flat surface on its side away from the gold or alloy layer. 12. The component of claim 11, wherein the gold alloy comprises gold and one or more of the following: copper, tin, cobalt, nickel, iron, indium, zinc, and molybdenum. 13. The component of claim 11, wherein the total weight of non-precious metals in the alloy is in the range of 0.001-50%. 14. The component of claim 11, wherein the base metal comprises one or more of nickel, cobalt, chromium, copper, and zinc. 15. The component of claim 11, wherein the minimum thickness of the base metal layer is in the range of 25-5000 micrometers.