JP2000007517A - Urethane (meth)acrylate-based dental adhesive composition - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the subject composition useful for the repair of a crown part of a tooth, concretely a hard resin tooth or the like by previously coating the position required to be repaired of the surface of a hardened body obtained by forming a resin matrix into a composite material with an inorganic filler, and excellent in toughness, adhesion permanency, and transparency by including a hardenable composition or the like. SOLUTION: This dental adhesive composition comprises (A) 5.0-0.0 wt.% hardenable composition obtained by substantially homogeneously mixing a polymer with a urethane (meth)acrylate not manifesting solubility and swelling properties to the polymer, (B) 35.0-95.0 wt.% polymerizable monomer having one or more reactive vinyl groups in one molecule, (C) 1.0-5.0 wt.% 4- acryloxyethyl trimellitate, and (D) the remainder of at least one kind of additives selected from an alkoxysilane compound, a polymerization initiator, a polymerization accelerator, an ultraviolet absorber, a fluorescent, a pigment and an opaquer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a durable adhesive containing the curable composition according to claim 1, which is obtained by reacting a hydroxyl group-containing (meth) acrylate in a homogeneous solution of a polymer and an isocyanate. It is a dental adhesive composition.

[0002] The present invention is a material for restoring a crown portion made of a composite, and more specifically, can be used as an adhesive for restoring a hard resin tooth or a hard resin front crown.

[0003]

2. Description of the Related Art Composites used for hard resin teeth are materials having excellent wear resistance and esthetics. However, when the surface layer decreases due to abrasion during denture production and long-term clinical wear. There is. However, the material of the composite was composed of a crosslinked polymer and an inorganic substance, and it was impossible to modify the surface with a solvent or the like. On the other hand, a method of treating the surface of the composite with an alkoxysilane compound and adhering it was also attempted, but did not reach practical use. In other words, there is no specific adhesive that restores a worn part and restores aesthetics or function. Even if a ceramic adhesive is used and surface treatment is performed and an adhesion test is performed, it is inferior in operability and durability and clinically It was difficult to use. An object of the present invention is to provide a repair method that can apply a light-curing type dental adhesive composition to a surface of a hard resin tooth where repair is required, and bond the hard resin pre-coated crown material or composite. .

[0004]

In order to solve the above problems, a method for repairing the surface of a hard resin tooth according to the present invention comprises the following components (a) and ( A dental adhesive composition consisting of b), (c) and (d) is applied, and a photopolymerization type, hard resin front covering or composite resin is to be built. The present invention is a dental adhesive excellent in toughness, adhesive durability, and transparency, which cannot be obtained by conventional adhesives by polymerization. More specifically, a curable composition in which a polymer and a urethane (meth) acrylate having neither solubility nor swelling property with respect to the polymer are homogeneously mixed.
Parts by weight, 4-acryloxyethyl trimellitic acid 1.0
To 10.0 parts by weight, and at least one additive component selected from a polymerization initiator, a polymerization accelerator, an ultraviolet absorber, a fluorescent agent, a pigment, and an opacifying agent. . The curable composition of the component (a) used in the present invention, in which the polymer and the urethane (meth) acrylate that does not exhibit solubility or swelling property with respect to the polymer are homogeneously mixed, has an adhesive durability. It is an essential component to obtain the property. This component is a composition in which urethane (meth) acrylate that does not dissolve or swell in the polymer is homogeneously mixed. Such a composition has high transparency, and its cured product has features such as an increase in crosslink density, a finer phase structure, and an increase in interphase adhesive strength.When used as a component of an adhesive, the bending properties of the cured adhesive are reduced. Improve its durability significantly.

Hereinafter, the dental adhesive of the present invention will be described in detail. The cured composition in which the polymer represented by the present invention and the urethane (meth) acrylate which does not exhibit solubility or swelling property with respect to the polymer are homogeneously mixed, and which is inherently soluble in polyalkyl methacrylate. Urethane (meth) acrylate, which does not show swelling, swells or dissolves homogeneously in polyalkyl methacrylate to form a highly viscous transparent mixture, in which polyalkyl methacrylate particles are not visually observed. , And in principle means that the polyalkyl methacrylate does not settle over time.

In the present invention, a composition in which a urethane (meth) acrylate having neither solubility nor swelling property with respect to polyalkyl methacrylate is homogeneously mixed is a homogeneous phase solution of polyalkyl methacrylate and a hydroxyl group-containing (meth) acrylate compound. Or a hydroxyl group-containing (meth) acrylate in a homogeneous phase solution of the polymer and the isocyanate compound by reversing the order of addition.

The resulting composition comprises urethane (meth) acrylate mixed homogeneously at the molecular level in polyalkyl methacrylate. Such a composition has high transparency and the cured product is characterized by an increase in crosslink density, a finer phase structure, an increase in interphase adhesive strength, and the like.

A polyalkyl methacrylate used for a composition in which a urethane (meth) acrylate which does not exhibit solubility or swelling property with respect to polyalkyl methacrylate is homogeneously mixed has an average molecular weight of 100,000 to 1,000,000 and an average particle size of 100,000 to 1,000,000. PMMA and PEMA of 1 to 75 μm. The polyalkyl methacrylate can be used with PEMA and / PMMA. These polyalkyl methacrylates can be obtained from hydroxyl-containing (meth) acrylate or trimethylhexamethylene diisocyanate (hereinafter referred to as "TMDI").
, Etc.), which can be dissolved or swelled by each of aliphatic isocyanates, alicyclic isocyanates, and aromatic isocyanate compounds. That is, when the polyalkyl methacrylate is mixed with a hydroxyl group-containing (meth) acrylate compound or an isocyanate compound, the polyalkyl methacrylate swells or dissolves uniformly to form a highly viscous transparent liquid mixture.

The hydroxyl group-containing (meth) used in the present invention
Acrylates are 2-HEMA, 3-hydroxypropyl methacrylate, 4-hydroxybutyl methacrylate, 5-hydroxypentyl methacrylate, 6-hydroxyhexyl methacrylate, 2-hydroxy-3-
Phenyloxypropyl methacrylate (hereinafter referred to as “2-
HPPA), 2-hydroxyethyl acrylate, 3-hydroxypropyl acrylate, and the like are suitable, preferably 2-HEMA, 2-HPPA, 3-hydroxypropyl methacrylate, and more preferably 2-hydroxyethyl acrylate.
HEMA, 2-HFPA.

On the other hand, as the isocyanate compound, T
MDI, hexamethylene diisocyanate (hereinafter referred to as “H
MDI), bisphenol A diisocyanate, dicyclohexyldimethylmethane diisocyanate, isophorone diisocyanate (hereinafter, “IPDI”)
), Tolylene diisocyanate, xylylene diisocyanate, diphenylmethane diisocyanate, naphthalene diisocyanate and the like are suitable, preferably TMDI, HMDI and IPDI. More preferred are TMDI and HMDI.

Further, a polyisocyanate having an isocyanate at a terminal obtained by reacting a polyol with an excess of diisocyanate can also be used. Such polyols include ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, 1,4-butanediol,
Examples thereof include 2,3-butanediol, 1,1,1-trimethylolpropane, and glycerin. The above-mentioned diisocyanates do not cause any problem.

A composition in which a urethane (meth) acrylate having neither solubility nor swelling property with respect to polyalkyl methacrylate is homogeneously mixed can be obtained by the following procedure. For example, a hydroxyl group-containing (meth) acrylate such as 2-HEMA is put into a flask, and nitrogen gas is blown into the flask.
The mixture is heated to 0 to 50 ° C., and the polyalkyl methacrylate is added little by little while stirring at a speed of 50 to 80 rpm to completely swell and dissolve.

Next, a tin catalyst generally used for urethane synthesis is dissolved, and after dissolution, the inside of the flask vessel is replaced with oxygen gas, and an isocyanate compound such as TMDI is dropped over 2 to 3 hours while blowing this gas. I do. Usually, a slight excess of diisocyanate is used. After dropping, 70 ±
The mixture was heated to 1 ° C to obtain the desired product.

Conversely, it is also possible to prepare TMDI or the like in a flask and add 2-HEMA or the like later, in the same manner as described above.

When a polyisocyanate having a terminal isocyanate obtained by reacting a polyol with an excess of diisocyanate is used, a polyhydric alcohol (having 2 to 4 hydroxyl groups) is reacted in a homogeneous phase solution of the polymer and the isocyanate. May be used by reacting the terminal isocyanate group with the hydroxyl group-containing (meth) acrylate.

The amount of the polymer is from 5.2 to 4 per mole of the isocyanate compound such as UDMA to be produced.
7.0 g is appropriate.

The alkoxysilane compound of the present invention is represented by the formula (I). YSiX4-n (I) In the formula, Y represents a hydrocarbon group or a vinyl-polymerizable reactive group, X represents a hydrolyzable group, and n represents a numerical value of 1, 2, or 3.

The general formula (I) YSiX4-n (I) used in the present invention, wherein Y represents a hydrocarbon group or a vinyl-polymerizable reactive group, X represents a hydrolyzable group, and n represents 1, 2 or 3]. Here, the hydrocarbon group represents a hydrocarbon group of an alkyl group having 1 to 3 carbon atoms or a phenyl group, or a mixture thereof. It also represents a vinyl-polymerizable reactive group, a vinyl group, an acryl group, a methacryl group or a mixture thereof. The hydrolyzable group has a property of being eliminated in an acid catalyst, and specific examples include an alkoxy group, a methoxyalkoxy group, an acetoxy group, and a phenyloxy group.

Examples of the silane compound represented by the general formula (I) include methyltriethoxysilane, ethyltrimethoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, methylphenyldiethoxysilane,
Trimethylmethoxysilane, trimethylethoxysilane, methoxyethyltriethoxysilane, acetoxyethyltriethoxysilane, methyltriacetoxysilane, methyltris (acryloxyethoxy) silane, methyltris (methacryloxyethoxy) silane, β-methacryloxyethyldimethoxymethylsilane, γ-acryloxypropylmethoxydimethylsilane, β-methacryloxyethyldimethoxymethylsilane, γ-methacryloxypropylmethoxydimethylsilane, γ-methacryloxypropyldimethoxysilane, γ-methacryloxypropyltrimethoxysilane, vinylmethyldimethoxysilane, vinyl Examples include trimethoxysilane, vinyltriethoxysilane, p-vinylphenylmethyldimethoxysilane, and the like.

CH3Si (OC2H5) 3 (I-1) C6H5Si (OCH3) 3 (I-2) CH2 = CHSi (OC2H40CH3) 3 (I-3) CH2 = CHSi (OCH3) 3 (I-4) CH2 = CHSi (OC2H5) 3 (I-5) CH2 = CCH3COOC3H6Si (OCH3) 3 (I-6)

The silane compound represented by the above structural formula is
Methyltriethoxysilane (I-1), phenyltrimethoxysilane (I-2), vinyltris (β-methoxyethoxy) silane (I-3), vinyltrimethoxysilane (I-4), vinyltriethoxysilane (I- 5), γ
-Methacryloxypropyltrimethoxysilane (I-
6).

These silane compounds may be used alone or in combination of two or more. Further, in one molecule, Y may include both an alkyl group and a vinyl-polymerizable reactive group, or a silane compound in which Y is an alkyl group and a silane compound in which Y is a vinyl-polymerizable reactive group may be used in combination. May be. Preferably, a silane compound in which Y is an alkyl group and a silane compound in which Y is a reactive group capable of vinyl polymerization are used in combination. More preferably, Y is a silane compound having a vinyl polymerizable reactive group.

[0023]

The present invention will be described below more specifically with reference to examples and comparative examples. [Preparation of a composition in which a urethane (meth) acrylate having neither solubility nor swelling property with respect to polyalkyl methacrylate) is homogeneously mixed] Urethane (meth) having neither solubility nor swelling property with respect to polyalkyl methacrylate
Composition 1 in which acrylate is homogeneously mixed 1. 260.3 g (2 mol) of 2-HEMA is placed in a glass flask with stirring blades, heated to 40 to 50 ° C. while blowing nitrogen gas, and stirred at a speed of 50 to 80 rpm. PMMA while
5.2 g was added little by little over 3 to 5 hours to completely swell and dissolve. 110 mg of dibutyltin dilaurate was added to the resulting solution. After the addition, the nitrogen gas was stopped and the inside of the flask was replaced with oxygen. While flowing oxygen gas, TM
210.3 g (1 mol) of DI was added dropwise over 2 hours.
After the completion of the dropwise addition, the mixture was heated to 70 ± 1 ° C., and the addition reaction was continued until all the isocyanate groups reacted to obtain a curable composition.
The reaction end point (hereinafter abbreviated as "B-1") was confirmed by an isocyanate equivalent titration method. The yield is 98.6%.
Met.

The reaction end point by the isocyanate equivalent titration method was measured by the following method. A 3 g sample is accurately measured in a stoppered Erlenmeyer flask. To this, 50 ml of di-n-butylamine solution is correctly added and left for 15 minutes.
Next, after adding 20 ml of reagent primary isopropyl alcohol, 1.5 ml of N / 10 sodium hydroxide solution was added to 0.1 g of bromcresol green indicator, and the mixture was ground well and dissolved. Add 3-4 drops and shake well. Then titrate with N / 2 hydrochloric acid. N / 2 near the end point
Hydrochloric acid is added dropwise and titration is continued while shaking the solution each time, and the point at which the blue or violet color disappears and the yellow color persists for at least 15 seconds is the end point. For this test, a blank test is performed under the same conditions.

[0025] ☆

(Equation 1)

A: The amount of N / 2 hydrochloric acid standard solution used in this test (ml) B: The amount of N / 2 hydrochloric acid standard solution used in blank test (ml) f: Factor of N / 2 hydrochloric acid standard solution S: Sample Amount collected (g)

A composition in which urethane (meth) acrylate, which does not show solubility or swelling property with respect to polyalkyl methacrylate, is homogeneously mixed. The composition is the same as in B-1 except that the amount of PMMA is 9.4 g. I got something. (Hereinafter abbreviated as "B-2") yield (99.5%).

A composition in which urethane (meth) acrylate, which does not show solubility or swelling with respect to polyalkyl methacrylate, is homogeneously mixed. PE instead of PMMA
A composition was obtained in the same manner as in B-1, except that 5.2 g of MA was used. (Hereinafter abbreviated as “B-3”) yield (99
%).

A composition in which a urethane (meth) acrylate having neither solubility nor swelling property with respect to polyalkyl methacrylate is homogeneously mixed, except that 9.4 g of PEMA is used, a composition is obtained in the same manner as in B-3. Was. (Hereinafter, abbreviated as "B-4") yield (98%).

A composition in which urethane (meth) acrylate, which does not exhibit solubility or swelling property with respect to polyalkyl methacrylate, is homogeneously mixed. 210.3 g of TMDI is added to a glass flask with stirring blades.
(1 mol), and while blowing nitrogen gas, 40 to 5
Heat to 0 ° C and stir at 50-80 rpm
9.4 g of EMA was added little by little over 3 to 5 hours to completely swell and dissolve. 110 mg of dibutyltin dilaurate was added to the resulting solution. After the addition, the nitrogen gas was stopped and the inside of the flask was replaced with oxygen. Then, while flowing oxygen gas, 260.3 g (2 mol) of 2-HEMA was added dropwise over 2 hours. After the dropwise addition, the mixture was heated to 70 ± 1 ° C., and the addition reaction was continued until all the isocyanate groups were reacted. The reaction end point is
A composition in which urethane (meth) acrylate, which was confirmed by FT-IR and isocyanate equivalent titration method and did not show solubility or swelling with polyalkyl methacrylate, was homogeneously mixed was obtained. (Hereinafter abbreviated as "B-5")
Yield (98.2%).

Composition 7 in which urethane (meth) acrylate, which does not show solubility or swelling with respect to polyalkyl methacrylate, is homogeneously mixed. 168.20 g (1 mol) of HMDI was placed in a glass flask with stirring blades,
The mixture was heated to 40 to 50 ° C. while blowing nitrogen gas, and 10 g of PEMA was added little by little over 3 to 5 hours while stirring at a speed of 50 to 80 rpm to completely swell. 110 mg of dibutyltin dilaurate was added to the resulting solution. After the addition, the nitrogen gas was stopped and the inside of the flask was replaced with oxygen. While flowing oxygen gas, 2-HFPA444.
5 g (2 mol) was added dropwise over 2 hours. 50 ± after dropping
The mixture was heated to 1 ° C., and the addition reaction was continued until all the isocyanate groups were reacted. 1,6-bis [(2-phenoxy-2′-acryloxy) isopropyl-oxy-carbonylamino] hexane (hereinafter abbreviated as “UDA”) ) Got.
The end point of the reaction was confirmed by FT-IR and isocyanate equivalent titration, and a composition was obtained in which urethane (meth) acrylate, which did not show solubility or swelling with polyalkyl methacrylate, was homogeneously mixed. (Hereafter, "B-
6 ")

A composition in which urethane (meth) acrylate, which does not show solubility or swelling with polyalkyl methacrylate, is homogeneously mixed. 504.6 g (3 mol) of HMDI is placed in a glass flask with stirring blades, and heated to 40 to 50 ° C. while blowing nitrogen gas, and the speed is increased to 50 to 50 ° C.
While stirring at 80 rpm, 9 g of PEMA is added little by little to 3 ~.
It was added over 5 hours to completely swell. 10 mg of dibutyltin dilaurate was added to the resulting solution. After the addition, the nitrogen gas was stopped and the inside of the flask was replaced with oxygen. While flowing oxygen gas, trimethylolpropane (hereinafter referred to as “T
134.18 g (1 mol) was added dropwise over 2 hours. After dropping, heat to 50 ± 1 ° C and add HMDI
Was subjected to an addition reaction between one isocyanate and TMP.

After the addition reaction, dibutyltin dilaurate 1
10 mg was added. After addition 2-HFPA666.75
g (3 mol) was added dropwise over 2 hours. 70 ± 1 after dripping
℃, the addition reaction is continued until all the isocyanate groups are reacted, trifunctional urethane acrylate oligomer 1,
1,1-tri [6 [(1-acryloxy-3-phenoxy) isopropyloxycarbanylamino] -hexylcarbamoyloxymethyl] propane (hereinafter “URO”
). The end point of the reaction was confirmed by FT-IR and isocyanate equivalent titration, and a composition was obtained in which urethane (meth) acrylate, which did not show solubility or swelling with polyalkyl methacrylate, was homogeneously mixed. (Hereinafter abbreviated as “B-7”) yield (98.5)
%).

A composition in which a urethane (meth) acrylate which does not exhibit solubility or swelling properties with respect to the polyalkyl methacrylates of B-1 to B-7 is homogeneously mixed is [FT]
-IR (FT-300)] (manufactured by Horiba, Ltd.) to measure the characteristic absorption of each, and to measure the average molecular weight and retention time of the polymer and the molecular weight and retention time of the urethane monomer using GPC. Then, it was confirmed that the respective compositions were obtained.

[Adhesion Test of Dental Adhesive] Example 2 60 parts by weight of the B1 composition obtained in the example, 40 parts by weight of EG, 2.0 parts by weight of γ-methacryloxypropyltrimethoxysilane, dimethylamino methacrylate Ethyl 1.
4 parts by weight, 3.0 parts by weight of 2-hydroxyethyl methacrylate, 0.7 parts by weight of camphorquinone, 3.0 parts by weight of 4-acryloxyethyl trimellitic acid, 0.035 of BHT
The dental adhesive was prepared by mixing parts by weight.

The adhesion test of the hard resin teeth and the adhesion test of the hard resin for the crown were performed by using a hard resin tooth endura postrio hard part (manufactured by Matsukaze Co., Ltd.) and hard surfaces with emery paper # 400, 800, 1000. After polishing stepwise, sandblasting was performed with alumina (average particle size: 50 μm) using a sandblaster (manufactured by Matsukaze Co., Ltd.). The processing conditions were a pressure of 3 kg / cm 2 and a time of 15 s. After treatment,
Washing with water for 5 minutes using an ultrasonic cleaner, 1% surfactant solution 5
After washing for 5 minutes and washing with water for 5 minutes, it was used after being air-dried. The adherend surface had a diameter of 5 mm. Visible light was irradiated for 60 seconds after uniformly applying the adhesive to the surface to be adhered. The number of test specimens was five.

The specimens were stored in water at 50 ° C. for 24 hours and 10 hours.
Cross head speed 1 using a universal testing machine [Autograph AG5000B] (manufactured by Shimadzu Corporation) after 3000 thermal cycles of storage in water for 5 days and 5 ° C and 60 ° C in water.
The tensile adhesive strength was measured at mm / min. The adhesive fracture surface was observed with an optical microscope. The results are shown in Table 1. All were cohesive failures of the adherend without any decrease in adhesive strength.

Example 3 A dental adhesive was prepared in the same manner as in Example 2 except that B2 was used in the B1 composition, and an adhesion test was performed. The results are shown in Table 1. All were cohesive failures of the adherend without any decrease in adhesive strength.

Example 4 A dental adhesive was prepared in the same manner as in Example 2 except that B3 was used for the B1 composition, and an adhesion test was performed. The results are shown in Table 1. All were cohesive failures of the adherend without any decrease in adhesive strength.

Example 4 A dental adhesive was prepared in the same manner as in Example 2 except that B4 was used in the B1 composition, and an adhesion test was performed. The results are shown in Table 1. All were cohesive failures of the adherend without any decrease in adhesive strength.

Example 5 A dental adhesive was prepared in the same manner as in Example 2 except that B5 was used for the B1 composition, and an adhesion test was performed. The results are shown in Table 1. All were cohesive failures of the adherend without any decrease in adhesive strength.

Example 6 A dental adhesive was prepared in the same manner as in Example 2 except that B7 was used for the B1 composition, and an adhesion test was performed. The results are shown in Table 1. All were cohesive failures of the adherend without any decrease in adhesive strength.

Example 7 A dental adhesive was prepared in the same manner as in Example 2 except that B8 was used for the B1 composition, and an adhesion test was performed. The results are shown in Table 1. All were cohesive failures of the adherend without any decrease in adhesive strength.

Comparative Example 1 An adhesive test specimen was prepared in the same manner as in Example 2 except that no dental adhesive was used, and an adhesive test was performed in the same manner. The results are shown in Table 1. The adhesive strength was extremely reduced, and all were interfacial peeling.

[0045]

[Table 1]

[0046]

[Table 2]

[0047]

[Table 3]

[0048]

The dental composition of the present invention is obtained by bonding a curable composition in which polymer molecules and a polymerizable monomer which does not swell or dissolve in the polymer are homogeneously mixed at a molecular level. By utilizing the tough mechanical properties, the adhesive has excellent durability.

Claims (8)

[Claims] 1. A dental adhesive composition comprising the following components a), b), c) and d) beforehand at a portion of the surface of a cured product in which a resin matrix is made into a composite material with an inorganic filler, where repair is required: a) 0.0% by weight of a curable composition in which a polymer and a urethane (meth) acrylate having neither solubility nor swelling property with respect to the polymer are homogeneously mixed. b) 35.0 to 95.0% by weight of a polymerizable monomer having at least one reactive vinyl group in one molecule. c) 4-acryloxyethyl trimellitic acid 1.0 to
5.0% by weight. d) At least one additional component selected from the group consisting of an alkoxysilane compound, a polymerization initiator, a polymerization accelerator, an ultraviolet absorber, a fluorescent agent, a pigment, and an opacifier. 2. The curable composition according to claim 1, wherein the polymer and a urethane (meth) acrylate having neither solubility nor swelling property with respect to the polymer are homogeneously mixed. 3. The curable composition according to claim 1, wherein the polymer is present in an amount of from 5.2 g to 47 g in 1 mol of the urethane (meth) acrylate. 4. The adhesive composition for a crown according to claim 1, wherein the urethane (meth) acrylate has at least one acryloyl group and / or methacryloyl group and urethane group in one molecule. 5. The curable composition according to claim 1, which is obtained by reacting an isocyanate in a homogeneous solution of a polymer and a hydroxyl group-containing (meth) acrylate. 6. The curable composition according to claim 1, which is obtained by reacting a hydroxyl group-containing (meth) acrylate in a homogeneous phase solution of a polymer and an isocyanate. 7. A polymer obtained by reacting a polyhydric alcohol (having 2 to 4 hydroxyl groups) in a homogeneous phase solution of a polymer and an isocyanate, and then reacting the reaction terminal isocyanate group with a hydroxyl group-containing (meth) acrylate. Item 10. The curable composition according to Item 1. 8. The dental adhesive composition according to claim 1, wherein the dental adhesive composition is applied to a portion of the composite surface where repair is required, and then the hard resin front covering material is laid.