JP2014148574A - Active energy ray-curable composition for forming hard coat layer, cured film-formation method, and laminate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an active energy ray-curable composition for forming a hard coat layer which can form a cured film excellent in film performance such as excoriation resistance with low coloration, a formation method of a cured film thereof, and a laminate having a cured film on the surface of a substrate.SOLUTION: In a cured film-formation method, an active energy ray-curable composition for forming a hard coat layer containing a siloxane oligomer (A) and an iodonium salt-based photoinitiator (B) represented by the general formula (1) is coated on a substrate, and then followed by irradiation with an active energy ray to form a cured film. A laminate has a cured film of the active energy ray-curable composition on the surface of a substrate.

Description

  The present invention relates to an active energy ray-curable composition for forming a hard coat layer, a method for forming the cured film, and a laminate having the cured film on the surface of a substrate.

In recent years, transparent plastic materials such as acrylic resin and polycarbonate resin, which are excellent in crush resistance and light weight, have been widely used as an alternative to transparent glass.
However, since the transparent plastic material has a surface hardness lower than that of glass, the transparent plastic material has a problem that the surface is easily damaged and a coloring problem such as yellowing of the cured film.
Thus, many attempts have been made to improve the scratch resistance and colorability of plastic materials.
For example, an active energy ray-curable composition or aromatic iodonium salt containing an iodonium salt-based polymerization initiator whose counter anion is hexafluoroantimonate or tetra (pentafluorophenyl) borate as a polymerization initiator (acid generator) An active energy ray-curable composition containing an acid generator is disclosed (for example, Patent Documents 1 to 3).

JP 2009-73944 A JP 2009-167243 A JP 2004-107562 A

  However, the cured film obtained from the curable compositions described in Patent Documents 1 to 3 can also be formed in a short time, but the problem that the scratch resistance is not sufficient, or the curing conditions to improve the scratch resistance. In this case, there is a problem that the cured film is easily colored.

  The present invention has been made in view of the above circumstances, and is an active energy ray-curable composition capable of forming an inorganic transparent cured film excellent in coating properties such as low coloration and scratch resistance in a short time, and its It is providing the laminated body which has the formation method of a cured film, and a cured film on the surface of a base material.

The first aspect of the present invention is:
The present invention relates to an active energy ray-curable composition for forming a hard coat layer, which contains a siloxane oligomer (A) and an iodonium salt photopolymerization initiator (B) represented by the following general formula (1).

The second aspect of the present invention is:
The siloxane oligomer (A) is a hydrolysis / condensation product of one or more compounds selected from alkoxysilanes represented by the following general formula (2) and alkylsilicates represented by the general formula (3). It relates to the active energy ray-curable composition.

(In the formula, R ¹ represents a monovalent organic group having 1 to 10 carbon atoms. R ² represents an alkyl group having 1 to 5 carbon atoms or an acyl group having 1 to 4 carbon atoms. A represents 0 to 3 carbon atoms. Represents any integer.)

^{(Wherein, R 3, R 4, R} 5 and ^{R 6} each independently represents an alkyl group having 1 to 5 carbon atoms .n represents an integer of 2 to 20.)

The third aspect of the present invention is:
Applying the active energy ray-curable composition to a substrate, and irradiating the active energy ray to form a cured coating film,
The fourth aspect of the present invention is:
The present invention relates to a laminate having a cured film of the active energy ray-curable composition on the surface of a substrate.

The active energy ray-curable composition of the present invention can be cured in a short time, and can be applied to a wide range of substrates and applications because a cured film having low coloration and excellent scratch resistance can be obtained. .

<Siloxane oligomer (A)>
The siloxane oligomer (A) used in the present invention is an oligomer having a siloxane bond as a main skeleton and a silanol group or an alkoxysilyl group that can be condensed using an acid generated from an acid generator by light irradiation as a catalyst. is there.
Silanol groups contained in the oligomer are condensed with silanol groups or alkoxysilyl groups to form siloxane bonds.
In addition, the alkoxysilyl group contained in the oligomer is condensed directly with the silanol group, or the alkoxysilyl group is once hydrolyzed to form a silanol group and then condensed with the silanol group or the alkoxysilyl group to form a siloxane bond. Can do.

Examples of the alkoxysilyl group include a methoxysilyl group, an ethoxysilyl group, and a propyloxysilyl group.
Among these, a methoxysilyl group or an ethoxysilyl group is preferable in terms of a high hydrolysis rate and condensation rate.

The molecular weight of the siloxane oligomer (A) is not particularly limited, but is 500 by weight average molecular weight.
-50,000 are preferable and 700-20,000 are more preferable.
By setting the mass average molecular weight of the siloxane oligomer (A) to 500 or more, the film formability of the active energy ray-curable composition can be improved, and the scratch resistance of the resulting cured film can be improved.
Moreover, the uniformity and transparency of the cured film obtained by making the mass average molecular weight of a siloxane oligomer (A) into 50,000 or less can be improved.
A siloxane oligomer (A) can be used individually by 1 type or in combination of 2 or more types.
As the siloxane oligomer (A), commercially available ones can be used as they are, and those synthesized by hydrolysis / condensation using various organosilanes, alkyl silicates and the like as raw materials can also be used.

In the present invention, as the siloxane oligomer (A), alkoxysilanes represented by the following general formula (2) and the following general formula (3) are used in that the boiling point of alcohol or carboxylic acid generated during hydrolysis or condensation is low. A hydrolyzate / condensate of one or more compounds selected from alkyl silicates represented by the formula:

(In the formula, R ¹ represents a monovalent organic group having 1 to 10 carbon atoms. R ² represents an alkyl group having 1 to 5 carbon atoms or an acyl group having 1 to 4 carbon atoms. A represents 0 to 3 carbon atoms. Represents any integer.)

^{(Wherein, R 3, R 4, R} 5 and ^{R 6} each independently represents an alkyl group having 1 to 5 carbon atoms .n represents an integer of 2 to 20.)

Examples of the alkoxysilanes represented by the general formula (2) include tetramethoxysilane, tetraethoxysilane, methyltrimethoxysilane, methyltriethoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, vinyltrimethoxysilane, Vinyltriethoxysilane, 3-methacryloyloxypropyltrimethoxysilane, 3-
Methacryloyloxypropyltriethoxysilane, 3-acryloyloxypropyltrimethoxysilane, 3-acryloyloxypropyltriethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropyltriethoxysilane, 2-
(3,4-epoxycyclohexyl) ethyltrimethoxysilane, 2- (3,4-epoxycyclohexyl) ethyltriethoxysilane, p-vinylphenylenetrimethoxysilane, p-vinylphenylenetriethoxysilane, dimethyldimethoxysilane and dimethyldi An ethoxysilane is mentioned.
Of these, tetramethoxysilane, tetraethoxysilane, methyltrimethoxysilane, methyltriethoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, dimethyldimethoxysilane are the fast hydrolysis and condensation reactions of alkoxysilanes. And dimethyldiethoxysilane are preferred.
Alkoxysilanes can be used alone or in combination of two or more.

Examples of the alkyl silicates represented by the general formula (3) include methyl silicate,
Examples include ethyl silicate, isopropyl silicate, n-propyl silicate, isobutyl silicate and n-butyl silicate.
Among these, methyl silicate in which all of R ^{3 to} R ⁶ represent a methyl group and ethyl silicate in which all of R ^{3 to} R ⁶ represent an ethyl group are preferable in that hydrolysis / condensation reaction of alkyl silicates is fast. .
Alkyl silicates can be used alone or in combination of two or more.

<Iodonium salt photopolymerization initiator (B)>
In the present invention, the iodonium salt photopolymerization initiator (B) is represented by the following general formula (1).

  By containing the iodonium salt-based photopolymerization initiator (B), it can be cured in a short time, and a cured film having low coloring and excellent scratch resistance can be obtained.

Although the compounding quantity of the said photoinitiator (B) is not specifically limited, From a viewpoint of curability and coloring suppression with respect to 100 mass parts of the said siloxane oligomer (A), 0.01-10 mass parts is preferable, 0.05-5.0 mass parts is more preferable.
Here, when the compounding amount of the photopolymerization initiator (B) is 0.01 parts by mass or more with respect to 100 parts by mass of the siloxane oligomer (A), the active energy ray-curable composition is irradiated with light for a short time. It tends to be cured to a good cured film.
Moreover, when the compounding quantity of the said photoinitiator (B) is 10 mass parts or less with respect to 100 mass parts of said siloxane oligomers (A), there is especially little coloring of the cured film obtained, and surface hardness of a cured film And the abrasion resistance tends to be good.

<Active energy ray-curable composition>
The active energy ray-curable composition of the present invention contains the siloxane oligomer (A) and the iodonium salt-based photopolymerization initiator (B), but the solid content is adjusted, the dispersion stability is improved, the coatability is improved, and the substrate. An organic solvent can be contained for the purpose of improving adhesion to the surface.
Examples of the organic solvent include alcohols, ketones, ethers, esters, cellosolves, and aromatic compounds.

Specific examples of the organic solvent include methyl alcohol, ethyl alcohol, 1-propyl alcohol, 2-propyl alcohol, 1-butyl alcohol, 2-butyl alcohol, isobutyl alcohol, t-butyl alcohol, benzyl alcohol, and 2-methoxyethyl alcohol. 2-ethoxyethyl alcohol, 2- (methoxymethoxy) ethyl alcohol, 2-butoxyethyl alcohol, furfuryl alcohol, tetrahydrofurfuryl alcohol, diethylene glycol, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, 1-methoxy- 2-propyl alcohol, 1-ethoxy-2-propyl alcohol, dipropylene glycol, dipropylene glycol Monomethyl ether, dipropylene glycol monoethyl ether, tripropylene glycol monomethyl ether, diacetone alcohol,
Acetone, methyl ethyl ketone, 2-pentanone, 3-pentanone, 2-hexanone, methyl isobutyl ketone, 2-heptanone, 4-heptanone, diisobutyl ketone, cyclohexanone, methylcyclohexanone, acetophenone, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether , Dihexyl ether, anisole, phenetole, tetrahydrofuran, tetrahydropyran, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, glycerin ether, Methyl acetate, ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, Isobutyl acetate sec- butyl acetate n- pentyl,
Isopentyl acetate, 3-methoxybutyl acetate, 2-ethylbutyl acetate, 2-
Ethylhexyl acetate, methyl propionate, ethyl propionate, butyl propionate, γ-butyrolactone, 2-methoxyethyl acetate, 2-ethoxyethyl acetate, 2-butoxyethyl acetate, 2-phenoxyethyl acetate, diethylene glycol monoethyl ether acetate, diethylene glycol Examples include monobutyl ether acetate, benzene, toluene and xylene. These organic solvents can be used individually by 1 type or in combination of 2 or more types.

The compounding amount of the organic solvent is 0 to 5,000 with respect to 100 parts by mass of the siloxane oligomer (A).
A mass part is preferable and 100-1,000 mass parts is more preferable. When the blending amount of the organic solvent is 1,000 parts by mass or less, the problem that the solid content becomes too low and the coating film becomes thin does not easily occur, and a cured coating film having good scratch resistance tends to be obtained.

In the active energy ray-curable composition of the present invention, various additives such as a polymer compound, colloidal silica, colloidal metal, leveling agent, ultraviolet absorber, light stabilizer, and antioxidant are added as necessary. Can be blended.

<Base material>
The active energy ray-curable composition of the present invention is applied to the surface of a substrate as a coating material, an ink material, or the like, and then cured by irradiation with active energy rays.
Examples of the base material used in the present invention include metal materials such as metal plates and metal cans, natural materials such as paper and wood materials, inorganic materials such as ceramics, plastic molded products such as acrylic resins and polycarbonate resins, and PET. And composite materials such as films of polyolefin and the like, electrodeposition-coated plates, and laminate plates.

Examples of the method of applying the active energy ray-curable composition of the present invention to the surface of a substrate include, for example, a spray method, a roll coater method, a gravure coater method, a flexo method, a screen method, a spin coater method, a flow coater method, Well-known methods, such as an electrostatic coating method, are mentioned.

<Hardened film>
In this invention, a cured film is formed by irradiating an active energy ray curable composition apply | coated to the surface of a base material with an active energy ray.
Examples of the active energy ray used in the present invention include vacuum ultraviolet rays, ultraviolet rays, and visible rays.

Specific examples of active energy rays include low-pressure mercury lamp, medium-pressure mercury lamp, high-pressure mercury lamp, ultra-high-pressure mercury lamp, incandescent lamp, xenon lamp, halogen lamp, carbon arc lamp, metal halide lamp, fluorescent lamp, tungsten lamp, gallium lamp, and excimer laser. And light using sunlight as a light source.
Among these, from the viewpoint of curability, light using a low pressure mercury lamp, a medium pressure mercury lamp, a high pressure mercury lamp, an ultrahigh pressure mercury lamp and a metal halide lamp as a light source is preferable.
An active energy ray can be used individually by 1 type or in combination of 2 or more types.

As the irradiation amount of the active energy ray, for example, in the case of irradiating ultraviolet rays, the integrated light amount is preferably 100 to 5,000 mJ / cm ² .
When the integrated light amount is small, curing is apt to be insufficient, and when the integrated light amount is large, coloring of the cured coating becomes conspicuous.

In the present invention, when the active energy ray-curable composition is cured by irradiation with active energy rays, it can be used in combination with heat curing as necessary.
As heating time when using heat curing together, if necessary, before active energy ray irradiation,
It is possible to select at least one time selected from any time after irradiation with active energy rays or after irradiation with active energy rays.

Examples of the heating method include an irradiation method using an infrared heater and a circulating heating method using hot air.
As a heating temperature, for example, from the viewpoint of curability, a temperature at which the temperature of the active energy ray-curable composition is 50 to 100 ° C. is preferable, and when heating after irradiation with active energy rays, the temperature of the cured product is 50 to 50 ° C. It is preferably 100 ° C.
Moreover, as heating time, from a sclerosing | hardenable viewpoint, when heating before active energy ray irradiation, for 1 to 20 minutes, when heating simultaneously with active energy ray irradiation, 0.2 to 10 minutes, and active energy ray irradiation When heating later, 1 to 60 minutes are preferable.
In the present invention, the thickness of the cured film is not particularly limited, but is usually 0.5 to 100 μm.
Degree.

<Laminate>
The laminate of the present invention is obtained by laminating a cured film of an active energy ray-curable composition on the surface of a substrate, and can be applied to various applications by selecting an appropriate substrate according to the application. it can.

EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention in detail, this invention is not limited to these Examples and a comparative example.
In the following description, “part” and “%” indicate “part by mass” and “% by mass”, respectively.
In addition, the evaluation method in an Example and a comparative example is as follows.

<Appearance evaluation>
The transparency of the laminate of acrylic plates having a cured coating formed on the surface, cracks, and the presence or absence of whitening were visually observed and evaluated according to the following criteria.
(Evaluation criteria)
○: Transparent and no cracking or whitening was observed.
X: The opaque part was recognized or the crack or whitening was recognized.

<Abrasion resistance evaluation>
Using a Taber abrasion tester (trade name: Rotary Ablation Tester, manufactured by Toyo Seiki Seisakusho Co., Ltd.), ΔH after 500 revolutions of abrasion was measured under the following conditions.
Wear wheel (trade name: CS10F (type IV)), rotational speed: 70 rpm, load: 4.9 N (500 gf), suction port height: 3 mm.
(Evaluation criteria)
○: ΔH is less than 20 ×: ΔH is 20 or more

<Pencil hardness>
The pencil hardness of the cured film was evaluated according to JIS-K5600-5-4; scratch hardness (pencil method).

<Base material adhesion>
After making the 100 squares on the cured film on the surface of the laminate of acrylic plates by cutting with a razor blade reaching the surface of the acrylic plate in 11 mm vertical and horizontal directions at intervals of 1 mm, the cellophane tape is adhered well Then, the film was peeled off rapidly in the 45-degree direction, and the number of squares remaining without the cured coating being peeled off was measured and evaluated according to the following criteria.
(Evaluation criteria)
◯: There are no peeled cells (good adhesion).
Δ: 1 to 5 peeled squares (medium adhesion).
X: Six or more squares which peeled (adhesion failure).

<Yellowness (YI)>
Yellowness (YI) is measured using NDR2000 (trade name) manufactured by Nippon Denshoku Industries Co., Ltd.
The yellowness of the cured film was determined by the following formula and evaluated according to the following criteria.
Yellowness of cured coating) = (yellowness of laminate of acrylic plates)-(yellowness of acrylic plates)
(Evaluation criteria)
○: Yellowness is 1.0 or less (good).
X: Yellowness exceeds 1.0 (defect).

[Synthesis Example 1: Synthesis of Siloxane Oligomer (A1)]
In a reaction vessel equipped with a stirrer, 24.52 g of methyltrimethoxysilane (manufactured by Shin-Etsu Chemical Co., Ltd., molecular weight 136.2), phenyltrimethoxysilane (manufactured by Shin-Etsu Chemical Co., Ltd., molecular weight 198.3) 3. 97 g and 8.5 g of isopropyl alcohol were added and stirred to obtain a uniform solution. Next, 21.6 g of water was added to the reaction vessel, followed by hydrolysis and condensation by heating at 80 ° C. for 9 hours while stirring. Furthermore, isopropyl alcohol was added to the reaction vessel to make the whole 73.2 g, and a solution of a siloxane oligomer (A1) having a solid concentration of 20% was obtained. When the mass average molecular weight by GPC of the siloxane oligomer (A1) was measured, the mass average molecular weight in terms of polystyrene was about 1,500.
In addition, in this invention, solid content concentration means the mass fraction computed with respect to the whole solution of the amount of the siloxane oligomer obtained when fully hydrolyzing and condensing.

[Synthesis Example 2] Synthesis of siloxane oligomer (A2) In a reaction vessel equipped with a stirrer, methyl silicate having a silica equivalent concentration of 51% (manufactured by Colcoat Co., Ltd., average tetramer, molecular weight (average value) 470.7, product Name: Methyl silicate 51) 9.
41 g, methyltrimethoxysilane (manufactured by Shin-Etsu Chemical Co., Ltd., molecular weight 136.2)
52 g and 6.9 g of isopropyl alcohol were added and stirred to make a uniform solution. Next, 26.6 g of water was added to the reaction vessel, and the mixture was heated at 80 ° C. for 3 hours with stirring to perform hydrolysis and condensation. Furthermore, isopropyl alcohol was added to the reaction vessel to make a total of 8
A solution of siloxane oligomer (A2) having a solid content concentration of 20% was obtained with 4.3 g. When the mass average molecular weight by GPC of the siloxane oligomer (A2) was measured, the mass average molecular weight in terms of polystyrene was about 4,300.

[Example 1]
(1) Preparation of active energy ray-curable composition 500 parts of a siloxane oligomer (A1) solution having a solid content concentration of 20% obtained in Synthesis Example 1 as a siloxane oligomer (A) in a container equipped with a stirrer (solid content As 100 copies)
2 parts of photopolymerization initiator (B) (tolylcumyl iodonium trifluoromethanesulfonate), 125 parts of PGM as a solvent, silicone surfactant as a leveling agent (trade name: L-7001, manufactured by Toray Dow Corning Co., Ltd.) One part was added and mixed to obtain an active energy ray-curable composition (1).

(2) Formation of coating film on surface of base material of active energy ray-curable composition As a base material, an acrylic plate (Mitsubishi Rayon (10 cm long, 10 cm wide and 3 mm thick))
Co., Ltd., trade name: Acrylite EX), and the thickness after drying is 3 to 4 μm.
After a suitable amount of the active energy ray-curable composition (1) prepared in the above (1) was dropped, it was applied by a bar coating method (using bar coater No. 26), and then heated by a hot air dryer. It dried for 10 minutes at 0 degreeC, and the coating film of the active energy ray hardening composition (1) was formed on the surface of the base material.

(3) Formation of a laminate of acrylic plates having a cured coating The substrate obtained in (2) above, on which a coating film of the active energy ray-curable composition (1) is formed, is provided with a conveyor. 120 W / cm high-pressure mercury lamp (Oak Manufacturing Co., Ltd. ultraviolet irradiation device, trade name: Handy UV-1200, QRU-2161 type)
Ultraviolet rays were irradiated so that the integrated light amount was 3,000 mJ / cm ² . Subsequently, it heated at 90 degreeC for 10 minute (s) with the hot air dryer, the cured film was formed on the surface of the acrylic board, and the laminated body of the acrylic board was obtained.
The integrated light quantity was measured using an ultraviolet light quantity meter (trade name: UV-351 type, manufactured by Oak Manufacturing Co., Ltd.).

(4) Evaluation of cured film Table 2 shows the results of various evaluations of the obtained cured film.

[Examples 2 to 4, Comparative Examples 1 to 4]
Preparation of active energy ray curable composition, active energy ray curable composition base material in the same manner as in Example 1 except that the siloxane oligomer (A) and the photopolymerization initiator listed in Table 1 are blended. Formation of a coating film on the surface, formation of a cured film, and various evaluations of the cured film were performed. Table 1 shows blending ratios used for various evaluations, and Table 2 shows various evaluation results.

A-1: Solution of siloxane oligomer (A1) having a solid content concentration of 20% obtained in Synthesis Example 1 A-2: Solution of siloxane oligomer (A2) having a solid content concentration of 20% obtained in Synthesis Example 2 PGM: 1- Methoxy-2-propanol L-7001: Silicone surfactant (manufactured by Dow Corning Toray, L-7001)
SI-100L (aromatic sulfonium salt): Photosensitive acid generator (manufactured by Sanshin Chemical Industry Co., Ltd., Sun-Aid SI-100L, solid content 50%)
Irg250: 75% propylene carbonate solution of iodonium, (4-methylphenyl) [4- (2-methylpropyl) phenyl] -hexafluorophosphate (1-) (manufactured by Ciba Specialty Chemicals Co., Ltd., trade name: IRGACURE250)
b-1: hexafluoroantimonate of photopolymerization initiator (B) b-2: triflate of triarylsulfonium salt

As shown in Table 1, the active energy ray-curable composition containing the iodonium salt photopolymerization initiator (B) having a specific structure obtained good results in various evaluations.
On the other hand, Comparative Examples 1 and 2 using a sulfonium salt photopolymerization initiator (SI-100L) as a photopolymerization initiator are iodonium salt photopolymerization initiators, but photopolymerization initiators having different counter anions are used. Comparative Examples 3 to 5 (Irg250 or b-1), Comparative Example 6 using a photopolymerization initiator (b-2) which is not an iodonium salt system and does not have a specific counter anion, are compared with Examples. In various evaluations, good results were not obtained.
As described above, according to the present invention, the active energy ray-curable composition capable of forming a cured film having low coloration and excellent film performance such as scratch resistance, a method for forming the cured film, and a cured film are provided on the surface of the substrate. It has been found that a laminate can be provided.

^{(Wherein, R 3, R 4, R} 5 and ^{R 6} each independently represents an alkyl group having 1 to 5 carbon atoms .n represents an integer of 2 to 20.)

Claims (4)

An active energy ray-curable composition for forming a hard coat layer, comprising a siloxane oligomer (A) and an iodonium salt-based photopolymerization initiator (B) represented by the following general formula (1).
The siloxane oligomer (A) is a hydrolysis / condensation product of one or more compounds selected from alkoxysilanes represented by the following general formula (2) and alkylsilicates represented by the general formula (3). The active energy ray-curable composition for forming a hard coat layer according to claim 1 or 2.

(In the formula, R ¹ represents a monovalent organic group having 1 to 10 carbon atoms. R ² represents an alkyl group having 1 to 5 carbon atoms or an acyl group having 1 to 4 carbon atoms. A represents 0 to 3 carbon atoms. Represents any integer.)

^{(Wherein, R 3, R 4, R} 5 and ^{R 6} each independently represents an alkyl group having 1 to 5 carbon atoms .n represents an integer of 2 to 20.)   A method for forming a cured film, comprising applying the active energy ray-curable composition according to claim 1 or 2 to a substrate and irradiating the active energy ray to form a cured film.   The laminated body which has the cured film of the active energy ray curable composition of Claim 1 or 2 on the surface of a base material.