JPH0647276B2 - Retort packaging container - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a retort (sterilization) packaging container, and more particularly, to a heat having a combination of pressure resistance and easy opening performance resistant to retort sterilization. Regarding sealed containers.

(Prior Art) Conventionally, a container formed by heat-sealing a flanged container body formed by drawing a metal foil sheet and a lid at a flange portion is widely used in many fields such as food packaging. .

The flanged container body is provided with an inner coating layer such as paint to prevent direct contact between the metal and the contents and to protect the metal, while the inner surface of the lid is for heat sealing purposes. An olefin resin inner surface material is provided.

Generally, in order to improve the adhesiveness between the coating film and the olefin resin, the acid-modified olefin resin is added to the coating film, and the acid-modified olefin resin is preferentially distributed on the surface of the coating film. It has already been known from JP-B-58-2825 to improve the adhesiveness.

(Problems to be Solved by the Invention) According to the above-mentioned prior art, there is an advantage that a strong adhesive bond can be introduced between the coating film and the olefin resin layer, but the heat seal between the lid and the flange is obtained. There is a problem that the seal strength is too strong for use as a so-called peeler boule seal container in which peeling is performed by hand at the interface to open the container.

As a solution to this problem, Japanese Patent Publication No. 58-385 discloses that a plurality of coating films are used for peeling between coating films, and Japanese Patent Publication No. 56-22699 discloses that acid modification in the coating film is performed. It is known that the distribution of the olefin resin is controlled by a distribution control agent layer therebelow to impart easy peeling property. However, these means have the trouble that the coating must be performed in two steps.

Therefore, an object of the present invention is to provide a sealing performance to withstand retort sterilization or a pressure-resistant performance and an easy-open performance, and to obtain an easily peelable performance on a heat-sealing surface between an acid-modified olefin resin-containing coating film and an olefin resin inner surface material The present invention provides a packaging container for retort which is designed to be used.

(Means for Solving the Problems) According to the present invention, a container body with a flange formed by drawing a metal foil sheet and a lid body are heat-sealed with the flange portion, The container body is provided with an epoxy-curing agent resin-based coating film containing an acid-modified olefin resin in an amount of 0.8 to 50 wt% per coating film as an inner surface coating layer, and the lid body is acid-modified as an inner surface material. The olefin unit constituting the olefin resin is 50 to 9
There is provided a retort packaging container characterized by comprising an olefin copolymer or olefin resin blend layer containing 5% by weight.

(Operation) The present invention uses a container body provided with an epoxy-curing agent resinous coating containing 0.8 to 50% by weight of an acid-modified olefin-based resin per coating as an inner surface coating layer and using an acid-modified olefin-based resin. When a lid provided with an olefin copolymer or olefin resin blend layer containing 50 to 95% by weight of the olefin unit constituting the above is used as an inner surface material and heat-sealed between the two, it is resistant to retort sterilization. However, it is based on the finding that a heat-seal bond is formed which can be easily opened by hand.

In the present invention, the acid-modified olefin resin-containing epoxy-curing agent resin-based coating film used as the inner surface coating material for the metal foil has both functions of forming a heat-sealing surface on the flange and protecting the metal foil. is there. Epoxy-curing agent resin-based coating film is one of the most excellent adhesiveness to metal foil and barrier property against corrosive components among various coating films, and has compatibility with acid-modified olefin-based resin. It is an indispensable ingredient because it has excellent properties. It is important that the epoxy-curing agent resin-based coating film contains an acid-modified olefin-based resin in order to provide heat sealability with the olefin-based resin inner surface material. That is, by only containing the olefin resin in the coating film, since no bond is formed between the olefin resin and the epoxy-curing agent resin constituting the coating film,
It will be understood that it is important to preliminarily acid-modify the olefin resin contained in the coating film.

In the present invention, it is also important to add the acid-modified olefin resin in an amount of 0.8 to 50% by weight, particularly 1.5 to 20% by weight, based on the coating film. If it is less, the heat seal strength with the inner surface material of the olefin resin tends to be weaker so that it cannot withstand retort sterilization, while if it is more than the above range, the adhesion of the coating film to the metal foil and corrosion resistance Not only does it decrease, but also the resistance to extraction decreases, such as an increase in the amount of potassium permanganate consumed, and the flavor retention with respect to the contents tends to decrease.

On the other hand, as the olefin resin inner surface material that is heat-sealed to the coating film, 50 to 95% by weight, particularly 65% by weight, of olefin units constituting the acid-modified olefin resin is used.
It is also important to use an olefin copolymer or an olefin resin blend containing from 85 to 85% by weight from the viewpoint of the sealing property to withstand retort or the combination property of pressure resistance and easy opening performance. That is, when the content of the olefin unit of the same kind as the olefin unit constituting the acid-modified olefin resin is less than the above range on the basis of the olefin copolymer or the olefin resin blend, the sealing performance to withstand the retort. If the pressure resistance is not obtained, and if it exceeds the above range, the heat-sealing strength tends to be too high, making it difficult to open by hand.

According to the present invention, as the olefin resin inner surface material which is heat-sealed to the acid-modified olefin resin-containing epoxy-curing agent resin coating, the same olefin unit as the acid-modified olefin resin is contained, and the olefin unit is By using the olefin copolymer or olefin resin blend layer having a content of a certain range as an inner surface material,
The heat-sealed portion having the retort resistance and the easy-opening property is successfully formed.

(Preferred Embodiment of the Invention) Packaging Container In FIG. 1 showing an example of the packaging container of the present invention, the packaging container comprises a container body 1 and a lid 2. Container body 1
Is a tapered or tubular body wall formed by drawing or forming a laminated or covering sheet described below, a bottom portion 3B continuous to the lower end of the body wall portion 3A, and a flange portion 4 continuous to the upper end of the body wall portion.
Made of. Second showing the cross-sectional structure of the container body sheet
In the figure, this sheet 5 is composed of a metal foil 6, an inner surface coating layer 7 applied to the surface of the metal foil which is the inner surface side of the container, and an outer surface coating protective layer 8 applied to the surface which is the outer surface side of the container.

In the present invention, as described above, the inner surface coating layer 7 is composed of an epoxy-curing agent resin coating film containing an acid-modified olefin resin.

On the other hand, the lid body 2 is composed of a laminated body as described below and has substantially the same size and shape as the outer periphery of the container body flange portion, and the lid body 2 and the flange portion 4 have a heat seal portion. 9 is formed. In FIG. 3 showing the cross-sectional structure of the laminated body constituting the lid body 2, the laminated body 1
Reference numeral 0 denotes a base layer 11 made of a metal foil or the like, an inner surface material layer 12 provided on the side of the base 11 that is the innermost surface of the container, and an outer surface protective layer 13 provided on the outer surface of the base layer 11.

As described above, the inner surface material layer 12 is an olefin copolymer or an olefin resin blend containing the same olefin units as the constituent olefin units of the acid-modified olefin resin present in the container inner surface coating layer 7 in a fixed amount ratio. Made of things.

The inner surface material layer 12 may be composed of a single layer of the above-mentioned olefin copolymer or olefin resin blend, but generally, the layer 12a of the above-mentioned copolymer or blend and the main olefin unit of this copolymer or blend. And a olefin homopolymer layer 12b having a common olefin unit are preferably laminated from the viewpoint of easy opening and prevention of feathering.

Container body As the metal foil that constitutes the container body, light metal foil such as aluminum foil, steel foil or iron foil, tin foil, chromic acid treated steel foil, electrolytic chromic acid treated steel foil, nickel plated steel foil, nickel Surface-treated steel foil such as tinned steel foil is used.
These metal foils are generally 0.3 to 0.02 mm, especially 0.2 to
It preferably has a thickness of 0.05 mm.

As the epoxy-curing agent resin-based coating material that constitutes the inner surface protective coating film, a coating material containing an epoxy resin and a curing agent resin corresponding thereto is the best. As the epoxy resin, a bisphenol type epoxy resin obtained by polycondensation of bisphenols such as bisphenol A and epihalohydrin is suitable, and the epoxy equivalent thereof is generally 400 to 2
It is preferably in the range of 0000, particularly 1000 to 5000. As a resin curing agent reactive with an epoxy resin, a functional group reactive with a hydroxyl group or an oxirane ring of the epoxy resin, for example, a hydroxyl group, an amino group,
Resins having carboxyl groups and the like; for example, resol type and / or novolac type phenol / formaldehyde resins, urea-formaldehyde resins, melamine / formaldehyde resins, alkyd resins, polyester resins, acrylic resins, polyurethane resins, xylene resins, epoxy ester resins, One or a combination of two or more such as butyral resin is used. Among these, a methylol group-containing thermosetting resin, particularly a resol-type phenol resin or an amino resin is preferable.

The ratio of the epoxy resin and the hardener resin is 95: 5 to 4
It is preferable that the weight ratio is 0:60, particularly 90:10 to 50:50.

As the acid-modified olefin resin, a resin obtained by graft-modifying an olefin resin with an ethylenically unsaturated carboxylic acid or an anhydride thereof is used. Examples of the olefin resin trunk polymer include low-medium- or high-density polyethylene, linear low density polyethylene, homopolypropylene, crystalline propylene-ethylene copolymer, polybutene-1, polypentene-1, butene-1 / propylene copolymer. Examples thereof include coalesce, butene-1 / propylene / ethylene terpolymer. From the viewpoint of retort resistance, homopolypropylene is preferable. Examples of the ethylenically unsaturated carboxylic acid or its anhydride include acrylic acid, methacrylic acid, maleic acid, fumaric acid, crotonic acid, itaconic acid, citraconic acid, 5-norbornene-2,3-dicarboxylic acid, maleic anhydride, and anhydrous. Citraconic acid, 5-norbornene-2,
Although 3-dicarboxylic acid anhydride, tetrahydrophthalic anhydride, etc. can be mentioned, maleic anhydride is most suitable from the viewpoint of heat-sealing property. The acid-modified olefin resin used preferably contains an acid group as a carbonyl group (= CO) at a concentration of 0.01 to 600 meq / 100 g resin, and particularly 1.0 to 200 meq / 100 g resin.

The inner coating film is formed by dispersing the acid-modified olefin resin in the epoxy-curing agent resin coating solution described above, applying the coating on a metal foil, and baking the coating.
The thickness of the coating film is generally 2 to 30 μm, especially 5 to 20 μm.
It is preferably in the range of μm. The baking of the coating is 180
C. to 250.degree. C. for 30 seconds to 10 minutes.

The protective coating provided on the outer surface of the metal foil is, for example, phenol
Formaldehyde resin, urea / formaldehyde resin,
Thermosetting resin consisting of melamine / formaldehyde resin, xylene / formaldehyde resin, epoxy resin, alkyd resin, polyester resin, thermosetting acrylic resin, urethane resin alone or in combination of two or more kinds: or acrylic resin, vinyl chloride-acetic acid It may be a coating film of a vinyl copolymer, a vinyl chloride-vinyl acetate maleic acid copolymer, a vinyl resin such as a vinyl butyral resin, a styrene-butadiene-acrylic acid ester copolymer, a thermoplastic resin such as a polyamide resin.

Molding into a container body can be obtained by subjecting the above-mentioned coated metal foil sheet to draw forming known per se, and subjecting the outer periphery of the flange portion to curling, if necessary. The drawing can be performed by using an arbitrary combination of a male die and a die (female die), but using a tool described in JP-B-56-50645 and a method described in JP-B-57-4408. Molding is preferable because wrinkles can be prevented from occurring near the root of the body wall at the flange.

Lid As the substrate of the lid, the metal foil described above; a thermoplastic resin film such as a biaxially stretched polyethylene terephthalate film, a biaxially stretched nylon film, a polycarbonate film; various papers; or a laminate of two or more of these is used. The inner surface material is applied to this to form a lid.

As already pointed out, the inner surface material used in the present invention contains the same olefin unit as the olefin unit of the acid-modified olefin resin and contains 50 to 95% by weight of this olefin unit.
In particular, the content is 65 to 85% by weight. That is,
The inner surface material used in the present invention contains 5 to 50% by weight of an olefin unit other than the olefin unit contained in the acid-modified olefin polymer in the form of a copolymer or a polymer blend.

For example, when the acid-modified olefin resin is maleic anhydride-modified polypropylene, the inner surface material has 5 propylene units.
It is contained in an amount of 0 to 95% by weight and contains 5 to 50% by weight of olefin units other than propylene, for example ethylene or butene-1, in the form of copolymers or blends. This also applies when the acid-modified olefin resin is acid-modified polyethylene.

In general, from the viewpoint of retort resistance, it is preferable that the inner surface material contains homopolypropylene or a propylene-ethylene copolymer mainly composed of a propylene unit, while the olefin unit different from the acid-modified olefin resin is a polymer blend. It is preferably contained in the inner surface material in a form. Thus, with respect to the acid-modified propylene resin, homopolypropylene or propylene-ethylene copolymer and polyethylene are used as the inner surface material at 50:50 to 95:
It is recommended to use blends in a weight ratio of 5, especially 60:40 to 80:20.

The thickness of the inner surface material layer is preferably 3 to 100 μm, particularly preferably 5 to 40 μm. When the polymer blend layer is used as the inner surface material, it is preferable to make the polymer blend layer as thin as possible within the heat sealable range in order to prevent the occurrence of feathering during opening. In order to enable this, in a preferred embodiment of the present invention, the inner surface material is a homopolyolefin having the above-mentioned polymer blend layer as a surface layer and a main olefin unit of the polymer blend layer as a constituent olefin unit, and a supporting layer. It has a laminated structure. With this configuration, it is possible to prevent the occurrence of feathering while ensuring reliable heat sealing with the polymer blend layer having a thickness of 3 to 10 μm and the homopolyolefin support layer having a thickness of 20 to 50 μm. Such a laminated inner surface material is easily manufactured by a coextrusion method known per se. The base material and the inner surface material can be joined or bonded easily by using the above-mentioned acid-modified olefin resin or urethane adhesive.

As the outer surface protective coating of the lid, the above-mentioned paint for the container body, various resin films, or the like is used.

Applications The packaging container of the present invention is provided in order to enhance the storability of contents.
Widely applied to containers that require sterilization operations such as retort sterilization. That is, after filling the contents of the container main body and performing operations such as nitrogen substitution and water vapor substitution as necessary, the container flange is covered with a lid, and heat such as heat seal bar, high frequency induction heating, and ultrasonic irradiation is applied. 170 using sealing means
Heat sealing is performed at a temperature of ˜250 ° C. for sealing. The hermetically sealed packaging container is sterilized at a temperature of 100 to 135 ° C. for 10 to 60 minutes to obtain a packaging body that can be stored. In addition, by opening the package and then placing it in an oven toaster, the contents can be heated and eaten.

(Effect of the Invention) According to the present invention, the acid-modified olefin resin-containing epoxy-curing agent resin-based coating contains the same olefin unit as the acid-modified olefin resin as the olefin resin inner surface material that is heat-sealed to the coating film. Moreover, by using the olefin copolymer or olefin resin blend layer having an olefin unit content within a certain range as the inner surface material,
It was possible to stably form the heat-sealed portion having the retort resistance and the easy opening property.

(Example) The present invention will be described in the following example.

Example 1 A yellow-colored epoxyphenol-based coating material was applied to one surface of a rolled steel foil having a chromate surface treatment layer on both sides and having a thickness of 75 μm at a coating amount of 65 mg / dm ² . On the other side, 100 parts by weight of epoxyphenol-based paint is isotactic, and the average carbonyl group concentration is 150 m · eq, where homopolypropylene is grafted with maleic anhydride.
A coating prepared by dispersing 6 parts by weight (5.66 wt%) of a powder prepared by adding aluminum hydroxide to modified polypropylene of 100 g / 100 g polymer was double coated and baked at 230 ° C. for 40 seconds. The coating amount after double coating was 160 mg / dm ² .

The material thus obtained was punched into a blank having a diameter of 135 mm and drawn by a drawing method using an elastic punch.
Height 25 mm, outer diameter 83 mm, inner diameter 70 mm, corner R25
In mm, a rectangular container having a flange portion curled outward was molded.

Further, as a lid material, a laminated base material of 12 μm PET / 15 μm biaxially stretched nylon / 20 μm aluminum foil and a thickness of 30
Between an ethylene-propylene copolymer support layer having a thickness of 5 μm and a coextruded film having a blend layer of 70 wt% polypropylene having a thickness of 5 μm and 30 wt% low density polyethylene, a 10 μm ethylene-propylene copolymer having a thickness of 15 μm maleic anhydride modified polypropylene ( After carrying out sand lamination by co-extruding two layers of resin with modified PP on the aluminum foil side, heat treatment was performed in an oven to obtain a laminate having the PP-LDPE blend layer as a seal layer. From this laminated body, a lid material having a size of 85 mm × 85 mm having a knob for opening at the corner was punched out.

Then, macaroni gratin was filled in the rectangular molding container, and the lid member was heat-sealed. This sealed container is 120 ℃
The retort sterilization process was performed for 30 minutes. The heat seal strength between the container body and the lid after the retort sterilization treatment was measured and found to be 2.5 kg / 15 mm. Further, when the lid material was opened from the opening knob, peeling could be easily performed.

On the other hand, the container was opened and the lid was removed, and the container body containing macaroni gratin was heated with an oven toaster of 890 W for 7 minutes. When the container was taken out from the oven toaster, it was markedly appropriate and the appearance was good. Also, when I tasted macaroni gratin, it was heated to the center and was very delicious.

Also, no strange odor was generated from the container body while heating the macaroni gratin with an oven toaster. In addition, no off-taste or off-odor was observed in the macaroni gratin after heating.

Examples 2 to 8 and Comparative Examples 1 to 3 Samples in which only the added amount of the maleic anhydride-modified PP of the modified polypropylene-added epoxyphenol-based paint used for coating the inner surface of the container body in Example 1 was changed as shown in Table 1. Each was prepared and the heat seal strength after retort treatment at 120 ° C. for 30 minutes was measured. The results are shown in Table 1.

In addition, the KMnO ₄ consumption of the container inner surface material (measurement method according to the operation of the Ministry of Health and Welfare Notification No. 20), which is very related to the flavor characteristics of the contents, was measured.

As in Comparative Examples 1 and 2, when the amount of modified PP added to the epoxyphenol-based material was very small, the heat sealing strength with the lid material was very weak, and the hermeticity could not be secured by the retort treatment.

Further, as in Comparative Example 3, when the amount of the modified PP added to the epoxyphenol-based paint was large, the KMnO ₄ consumption was large, and the flavor of the contents tended to be deteriorated.

On the other hand, if the amount of modified PP added is as in Examples 2 to 8, the heat sealing strength with the lid material is about 2.5 kg / 15 mm,
The retort resistance was sufficient and peeling was possible. The flavor characteristics were also good.

The inner surface coating was a double coat, but in Example 8, an epoxyphenol-based paint containing no modified PP was used for the first coat, and the second coat is shown in Table 1. A paint was used in which an amount of modified PP was added.

Examples 9 to 12 By the same method as in Example 1, container bodies shown in Table 2 were produced.

In addition, a lid material as shown in Table 3 was prepared by the same method as in Example 1, and after filling the contents shown in Table 3, the third material was filled.
Retort treatment was performed under the conditions shown in the table. In all cases, there was no change in the container due to the retort treatment, and the sealing property was also perfect. When the heat seal strength after the retort treatment was measured, it was about 2.5 kg / 15 mm as shown in Table 3, and opening by peeling was possible.

The inner coating material was applied by double coating so that pinholes in the coating film did not occur. All coating amount is 160mg
/ dm ² ± 5 mg / dm ² .

Comparative Example 4 In Example 1, as a sealing material for the lid material, a copolymer having an LDPE-rich sealing layer composed of a 30 μm ethylene-propylene copolymer support layer / 5 μm low density polyethylene 70 wt% and a polypropylene 30 wt% blend layer was used. When extruded film was used, the heat seal strength before retort was very weak at 0.6 Kg / 15 mm, 120 ° C 3
When the retort treatment was performed for 0 minutes, a sample in which the seal portion peeled off was seen.

Example 13 A brown-colored epoxyphenol-based coating material was applied to one surface of a rolled steel foil having a chromate surface treatment layer on both sides and having a thickness of 75 μm at a coating amount of 65 mg / dm ² .

Next, an epoxyphenol-based coating material containing a bisphenol A type epoxy resin and a resole type phenol-formaldehyde resin containing a binuclear component in a weight ratio of 95: 5 was applied to the other surface as a base coat and dried.
The coating amount of the base coat was 120 mg / dm ² . Next, use epoxy resin as a top coat on the surface of this base coat.
Benzoguanamine-based paint (specific gravity d ₁ = 1.20 g / cm ³ ) 10
0 parts by weight of isotactic homopolypropylene maleic anhydride grafted polymer modified polypropylene having an average carbonyl group concentration of 150 meq / 100 g to which aluminum hydroxide was added (specific gravity d ₂ = 0.90 g / Cm ³ ) 6 parts by weight (5.66 wt%) of the coating composition was applied and baked at 250 ° C. for 40 seconds. The coating amount of the top coat layer was 70 mg / dm ² .

The thermal softening temperature of the obtained inner coating film was measured by a thermomechanical analyzer (T
(MA) is measured by the venelation method.
It was 10 ° C.

The material thus obtained is blanked into a blank of φ135 mm and drawn by an elastic material punch to obtain a height of 25 mm, an outer diameter of 83 mm, an inner diameter of 70 mm and a corner R.
A rectangular container having a diameter of 25 mm and a flange portion curled outward was molded.

Further, as a lid material, a laminated base material of 12 μm PET / 15 μm biaxially stretched nylon / 20 μm aluminum foil and a thickness of 3
Between a 0 μm ethylene-propylene copolymer support layer and a coextruded film consisting of a blend layer of 70 μm polypropylene having a thickness of 5 μm and 30 wt% low density polyethylene, a 10 μm ethylene-propylene copolymer 15 μm maleic anhydride modified polypropylene ( After carrying out sand lamination by co-extruding two layers of resin with modified PP on the aluminum foil side, heat treatment was performed in an oven to obtain a laminate having the PP-LDPE blend layer as a seal layer. From this laminated body, a lid material having a size of 85 mm × 85 mm having a knob for opening at the corner was punched out.

Then, macaroni gratin was filled in the rectangular molding container, and the lid member was heat-sealed. This sealed container 12
The retort sterilization treatment was performed at 0 ° C. for 30 minutes. The heat seal strength between the container body and the lid after the retort sterilization treatment was measured and found to be 2.5 kg / 15 mm. When the lid material was opened from the opening knob, peeling could be easily performed.

Further, the container was opened, the lid was removed, and the container body containing macaroni gratin was heated with an oven toaster of 890 W for 7 minutes. When I took out the container from the oven toaster,
The macaroni gratin had a moderate amount of focus. When I tried this macaroni gratin, it was heated to the center and was very delicious. On the other hand, no change such as discoloration was observed in the flange portion of the container body where the temperature rises most when heated with an oven toaster. In addition, when the macaroni gratin was attached to the inner surface of the container body, it was not found to be attached.

Further, no abnormal odor was generated from the container body while heating the macaroni gratin with an oven toaster. In addition, no off-taste or off-odor was observed in the macaroni gratin after heating.

[Brief description of drawings]

FIG. 1 is a perspective view showing an example of the packaging container of the present invention, FIG. 2 is a cross-sectional view showing an example of the container body sheet of the present invention, and FIG. 3 shows an example of a lid of the present invention. FIG. 2 is a cross-sectional view of a laminated body in which reference numeral 1 is a container body, 2 is a lid, 3A is a body wall portion, and 3B
Is a bottom part, 4 is a flange part, 5 is a container main body sheet, 6 is a metal foil, 7 is an inner surface coating layer, 8 is an outer surface coating protective layer, 9 is a heat seal part, 10 is a laminate constituting a lid, 11 is A substrate, 12 is an inner surface material layer, and 13 is an outer surface protection layer.

Claims (3)

[Claims] 1. A container obtained by heat-sealing a container body with a flange formed by drawing a metal foil sheet and a lid body at the flange portion, wherein the container body is made of an acid-modified olefin resin. 0 per coating.
An epoxy-curing agent resin-based coating film containing 8 to 50% by weight is provided as an inner surface coating layer, and the lid body contains 50 to 95% by weight of an olefin unit constituting an acid-modified olefin resin as an inner surface material. A retort packaging container comprising an olefin copolymer or an olefin resin blend layer contained therein. 2. A polymer in which the acid-modified olefin resin is an acid-modified polypropylene resin, the inner surface material contains polypropylene or a propylene-ethylene copolymer and polyethylene, and the content of propylene units is 50 to 95% by weight. The packaging container according to claim 1, which is a blend. 3. The inner surface material has the blend layer on the surface of 2 to 1
The packaging container according to claim 2, comprising a polypropylene film having a thickness of 0 to 100 μm and a thickness of 15 to 100 μm.