JP5112570B1 - Coating method and curing agent for polyurethane paint - Google Patents

Abstract

In a coating method using a two-coat one-bake method in which a base coat paint is applied and then a clear coat paint is applied and the two coating films are simultaneously cured, compared to the conventional method, the distance between the two coating films is reduced. Increase adhesive strength.
A specific curing agent is used in a clearcoat paint. A coating method in which the curing agent includes a polyisocyanate derived from 1,6-hexamethylene diisocyanate and satisfies all of the following conditions:
1) the curing agent is substantially free of diisocyanate monomer,
2) The viscosity of the curing agent is 1000 mPa · s / 23 ° C. or less.
3) The content of the HDI trimer in the curing agent is 60% by weight or more, and 4) The content of the HDI dimer in the curing agent is less than 10% by weight.
[Selection figure] None

Description

  The present invention relates to a coating material having excellent physical properties such as high adhesion strength between coating films and high water resistance in a 2-coat 1-bake coating method for coating on steel plates and plastics of automobiles and home appliances.

  In the coating of steel plates and plastics for automobiles and home appliances, for the purpose of shortening the painting process and saving energy, the so-called wet-on-wet method or the two-coat one-bake method (in some cases, three-coat one-bake method) is used. The method is widely adopted. In these coating methods, for the purpose of reducing VOC (volatile organic compounds) at the same time, a low VOC paint such as an aqueous paint is often used as a paint for the base layer and the intermediate coating layer.

  However, since the paint used for the base layer and the intermediate coating layer does not reach the level of the two-component reactive coating film in terms of coating film performance, problems such as poor adhesion, solvent resistance, and water resistance occur. There has been a demand for a method for improving the disadvantages.

  In Patent Documents 1 and 2, as a base coat, a paint composed of a resin containing a hydroxyl group or a carboxyl group and a catalyst is applied, and then a clear coat mainly composed of a hydroxyl group-containing resin and a polyisocyanate compound is applied to form isocyanate in the clear coat paint. A method is described in which the compound is partially penetrated into the base coat and cured.

  In Patent Document 3, 1,6-hexamethylene diisocyanate (hereinafter, sometimes referred to as HDI) is used as a curing agent for the clear coat in order to partially transfer the isocyanate compound in the clear coat to the base coat. It is described that a polyisocyanate containing 1% by weight or more and 70% by weight or less of a dimer (also called uretdione) is used in an NCO / OH ratio range of 1.2 to 3.0.

JP 61-161179 A JP-A-9-271714 JP 2001-226626 A

  However, in the methods of Patent Document 1 and Patent Document 2, there is a case where an isocyanate compound sufficient to crosslink the base coat does not penetrate, and there are cases where there are problems in adhesion between the coating layers and solvent resistance. Further, in the method of Patent Document 3, since the dimer of 1,6-hexamethylene diisocyanate has an isocyanate functionality of 2, the degree of cross-linking of the coating layer is low, and there is a problem of poor solvent resistance, weather resistance, and the like. .

  By using a specific polyisocyanate as a curing agent in the clearcoat paint, the present inventors efficiently and well penetrate the polyisocyanate in the clearcoat into the lower basecoat and react with the isocyanate-reactive component in the basecoat. For this reason, the present inventors have found that the adhesive strength between the clear coat layer and the base coat layer is significantly increased, thereby completing the present invention.

Aspects of the present invention are as follows.
[Claim 1]
A base coat paint containing an active hydrogen-containing resin is applied, and this is not cross-linked, and then a clear coat paint containing a curing agent and an active hydrogen-containing resin is applied on the base coat, and then the base coat and the clear coat are simultaneously cured by cross-linking. A two-coat one-bake method of coating, wherein the clear-coat paint is a two-component polyurethane paint having an NCO / H (active hydrogen) molar ratio of 0.9 to 2.0, A coating method in which the curing agent contains a polyisocyanate derived from 1,6-hexamethylene diisocyanate and satisfies all of the following conditions.
1) Curing agent is substantially free of diisocyanate monomer (monomer) 2) Viscosity of curing agent is 1000 mPa · s / 23 ° C. or less 3) Trimer of 1,6-hexamethylene diisocyanate in curing agent 4) The content of the dimer of 1,6-hexamethylene diisocyanate in the curing agent is less than 10% by weight [Item 2]
Item 2. The method according to Item 1, wherein the curing agent for clear coat paint contains 13 mol% or more of iminooxadiazinedione ring of 1,6-hexamethylene diisocyanate.
[Section 3]
Item 3. The method according to Item 1 or 2, wherein the base paint contains an active hydrogen-containing resin and a blocked isocyanate.
[Claim 4]
Item 4. A curing agent for clear coat paint used in the method according to any one of Items 1 to 3.
[Section 5]
The multilayer coating film which consists of a basecoat layer and a clearcoat layer obtained by the method in any one of claim | item 1-3.

  Adhesive strength between the base coat layer and the clear coat layer is greatly improved, and excellent film properties (for example, excellent solvent resistance, water resistance, and adhesiveness) can be obtained. This is believed to be because the clearcoat polyisocyanate penetrates the basecoat efficiently and well and reacts with the isocyanate-reactive components in the basecoat.

  Hereinafter, the present invention will be described in detail.

  The coating method according to the present invention comprises: applying a base coat paint containing an active hydrogen-containing resin to form a base coat; cross-linking and curing the base coat; and then a clear coat paint containing a curing agent and an active hydrogen-containing resin on the base coat This is a two-coat one-bake coating method in which a clear coat is formed by coating (two-component reactive polyurethane paint), and the base coat and the clear coat are simultaneously crosslinked and cured. This coating method is sometimes called a wet-on-wet system. Further, depending on the case, the coating method of the present invention includes forming the base coat into two layers to form a three-coat one-bake method or more layers.

After coating the base coat, in order to avoid a mixed layer with a clear coat to be applied later, the volatile matter (for example, solvent and water) contained in the base coat is volatilized to some extent, and the touch is dried by JIS K 5600-1-1. Or it is preferable to dry to the extent of semi-hardening. If it is a highly volatile solvent, it is about 1 to 10 minutes (particularly about 3 minutes) at room temperature, and if it contains a low-volatile solvent or water, it is about 50 to 100 ° C for about 1 to 10 minutes (particularly about 3 minutes). Drying takes place.
The heating conditions after the clearcoat coating are generally about 1 to 3 hours (particularly about 10 to 60 minutes) at a temperature of about 80 to 180 ° C. When the heating conditions are limited, the clear coat coating curing agent of the present invention is reactive at room temperature, and can be cured at room temperature for a long time without heating.

  The present invention relates to a clear coat curing agent for forming a polyurethane multilayer coating film and a formed polyurethane resin coating film in this coating method. As a result of intensive studies on an effective curing agent in this coating method, the inventors have selected a specific polyisocyanate as a curing agent for the clear coat, so that the curing agent sufficiently penetrates to the base coat and further crosslinks. As a result, it was discovered that the obtained polyurethane multilayer coating film can form coating film properties including excellent adhesion, and the present invention has been completed.

  The base coat paint comprises an active hydrogen-containing resin. The clear coat paint (two-component reactive urethane paint) comprises a curing agent and an active hydrogen-containing resin.

The specific curing agent referred to here is a polyisocyanate that satisfies all of the following conditions.
The curing agent includes a polyisocyanate derived from 1,6-hexamethylene diisocyanate (HDI) alone, and may include a polyisocyanate derived from an aliphatic or alicyclic diisocyanate other than HDI. And
1) the curing agent is substantially free of HDI diisocyanate monomer,
2) The viscosity of the curing agent is 1000 mPa · s / 23 ° C. or less.
3) The content of the HDI trimer in the curing agent is 60% by weight or more, and 4) The content of the HDI dimer in the curing agent is less than 10% by weight.

  The curing agent is a polyisocyanate derived from diisocyanate. The curing agent is mainly composed of a polyisocyanate derived from an HDI monomer (HDI monomer). The polyisocyanate synthesized from HDI is 60% by weight or more, preferably 70% by weight or more, more preferably 90% by weight or more, for example, 95% by weight or more based on the curing agent. Polyisocyanates derived from diisocyanate monomers other than HDI (eg, aliphatic or alicyclic diisocyanate monomers such as isophorone diisocyanate, dicyclohexylmethane 4,4′-diisocyanate, norbornene diisocyanate) also satisfy the conditions of the curing agent of the present invention. It can be used within the range.

As a secondary raw material for synthesizing polyisocyanate from diisocyanate monomer (generally, 30% by weight or less of curing agent), catalysts, promoters, active hydrogen compounds, reaction terminators used in polyisocyanate synthesis known in the industry And may be used as necessary.
It is also possible to mix solvents that do not react with the polyisocyanate (such as ethyl acetate, butyl acetate, toluene, xylene, solvent naphtha) in the curing agent, but from the viewpoint of reducing VOC, it is preferable not to use solvents. .

  In order to industrially use the polyisocyanate of the present invention, it is preferable that a diisocyanate monomer (diisocyanate monomer) is not substantially contained from the viewpoint of occupational health. The amount of diisocyanate monomer contained in the polyisocyanate is preferably 1% by weight or less, more preferably 0.5% by weight or less, and particularly preferably 0.2% by weight or less. Usually, this condition is satisfied by the step of removing the diisocyanate monomer from the crude product obtained after completion of the reaction under reduced pressure.

  The polyisocyanate used in the present invention is an HDI trimer as a main component. This trimer includes the isocyanurate structure of HDI, the iminooxadiazinedione structure of HDI, or both.

The term “trimer of HDI” in the present invention means a polyisocyanate having an isocyanurate group formed by adding three molecules of HDI or an iminooxadiazinedione group which is an isomer structure of an isocyanurate group. In the industry, biuret groups and allophanate groups have been widely used as polyisocyanate components made of HDI, but the formation of these biuret groups and allophanate groups involves CO ₂ elimination and addition of alcohols. In the present invention, it is not called an HDI trimer. Uretodione is a dimer formed by adding two molecules of HDI.

  In the actual synthesis of polyisocyanate, a composition on the polymer side exceeding the trimer is often generated at the same time. For example, an HDI pentamer or heptamer having an isocyanurate group can be considered, but these are isocyanurate structures, but naturally they are not trimers, and therefore do not correspond to the HDI trimer according to the present invention.

  If the manufacturing method of the polyisocyanate derived from 1, 6-hexamethylene diisocyanate (HDI) used by this invention is a manufacturing method which produces | generates an isocyanurate group and / or iminooxadiazine dione group using HDI. Any method may be used. For example, known methods described in JP-A-11-152320, JP-A-2000-086640, JP-T 2004-534870, US2011 / 0281965 and the like can be mentioned.

  Specific examples of the production method of polyisocyanate include oligomerization in the presence of a hydrogen polyfluoride oligomerization catalyst (JP-A-11-152320), quaternary ammonium and phosphonium fluoride trimerization. Method of trimerization in the presence of a catalyst (Japanese Patent Laid-Open No. 2000-086640) Trimerization in the presence of a catalyst which is a salt-like compound having a 1,2,3- and / or 1,2,4-triazolate structure The method (Japanese translations of PCT publication No. 2004-534870 gazette) is mentioned.

  If the amount of the HDI trimer is 60% by weight or more based on the curing agent, the penetration of the polyisocyanate from the clear coat to the base coat occurs sufficiently, and the functionality is 3. Is enough.

  In the curing agent, the amount of iminooxadiazinedione ring of HDI is 13 mol% or more, particularly 25 mol% or more, for example 30 to 80 mol%, based on the total of isocyanurate ring and iminooxadiazinedione ring. Preferably there is. In the present invention, it has been found that the iminooxadiazinedione structure can particularly contribute efficiently to lowering the viscosity of the curing agent, improving the permeability to the lower layer, and improving the physical properties of the coating film.

Hardener is
The HDI dimer is less than 10% by weight in the curing agent, and the viscosity of the curing agent is 1000 mPa · s / 23 ° C. or less,
It is necessary to satisfy the conditions.

  If the amount of the dimer of HDI exceeds 10% by weight with respect to the curing agent, the dimer (uretdione) is bifunctional, so the crosslinkability of the coating film is poor, and the water resistance and interlayer adhesion are poor. . When the viscosity exceeds 1000 mPa · s / 23 ° C., the proportion of the clear coat polyisocyanate transferred to the base coat becomes small (it is assumed that the higher molecular weight polyisocyanate inhibits penetration into the lower layer). Does not exhibit sufficient performance in terms of water resistance and adhesion.

  As components other than HDI trimers and dimers, there are polyisocyanates having the aforementioned biuret groups and allophanate groups, but polyisocyanates having a biuret structure as a main component have a viscosity of 1000 mPa · s / 23 ° C. or higher. Thus, the penetration into the lower layer (base coat) becomes insufficient. The polyisocyanate having an allophanate structure as a main component can reduce the viscosity, but the functionality decreases as in the case of the dimer, so that the cross-linking property of the coating film is poor, and the water resistance and interlayer adhesion are poor. Also, the degree of penetration into the lower layer is low.

  When the conditions of the curing agent of the present invention are not satisfied, the water resistance of the coating film, the interlayer adhesion, etc. cannot be sufficiently obtained because the functionality is lowered or the penetration level into the lower layer is insufficient.

  On the other hand, various modified polyisocyanates other than HDI trimers and dimers can be used as long as they satisfy the conditions of the curing agent of the present invention. Various modified types of polyisocyanates may be mixed to prepare a curing agent that satisfies the conditions of the curing agent of the present invention.

  The ratio of the polyisocyanate used in the present invention to the active hydrogen-containing resin used in the clear coat is 0.9 to 2.0, particularly in a molar ratio represented by NCO / H (isocyanate group / active hydrogen atom). A range of NCO / H ratio of 0.9 to 1.2 is preferred. More preferably, the NCO / H molar ratio is in the range of 1.0 to 1.2.

  When the molar ratio is less than 0.9, the amount of polyisocyanate transferred from the clear coat to the base coat decreases, so that sufficient crosslinking cannot be formed, and water resistance, adhesiveness, and the like are lowered. Since the polyisocyanate of the present invention has high functionality and can form a high-performance coating film with a small amount of penetration, an excessive NCO / H molar ratio is not required. On the other hand, when it exceeds 2.0, the reactivity delay by excess polyisocyanate occurs and it is not preferable economically. An NCO / H molar ratio of 0.9 to 1.2 can exhibit sufficiently high coating film properties.

The resin used for the base paint comprises a resin containing an isocyanate-reactive active hydrogen. Specific examples include resins having a hydroxyl group or a carboxyl group. Acrylic resins, polyester resins, alkyd resins, polyurethane resins and the like containing active hydrogen used in the industry can be used. In particular, from the viewpoint of VOC reduction, a water-dispersible hydroxyl group-containing acrylic resin is suitable. For example, the active hydrogen content is preferably 0.01 to 2% by weight. In this specification, “active hydrogen content” means the weight ratio (% by weight) of active hydrogen atoms to the weight of the resin. In general, the hydroxyl group content is 0.1 to 15% by weight, preferably 0.2 to 10% by weight. Generally, a resin having an acid value of 0 to 60 mg KOH / g, preferably 0 to 40 mg KOH / g, and a number average molecular weight of 500 to 100,000, preferably 5,000 to 300,000 is used.
A combination of these resins and melamine resins can also be used. As the melamine resin, an alkyl etherified melamine resin suitable for water-solubilization is also preferable. The mixing ratio of these active hydrogen-containing resins and melamine resins is preferably 100: 3 to 60 in terms of weight ratio of nonvolatile components.

  As the base paint, an active hydrogen-containing resin alone or a one-pack paint used in combination with a block polyisocyanate as required is used. As the block polyisocyanate, oximes that are known blocking agents are polyisocyanates having any of biuret, isocyanurate, urethane, uretdione, and allophanate groups obtained from 1,6-hexamethylene diisocyanate and / or isophorone diisocyanate, It is blocked with a lactam, an active methylene, a pyrazole compound or the like, and can be blocked with a compound such as 2-butanone oxime, ethyl acetoacetate, diethyl malonate, or dimethylpyrazole, for example. These block polyisocyanates can be arbitrarily selected according to the baking conditions.

  When using a block polyisocyanate, the ratio of the active hydrogen-containing resin to the block polyisocyanate is preferably 0.8 or less in terms of an effective NCO / H molar ratio. Base coat paints include solid color paints using inorganic and organic pigments commonly used in the field, metallic paints such as scale-like aluminum, metallic paints using mica-like iron oxide, and pearl paints. You can also

  Basecoat paint resins include cure catalysts used in the art, such as organic, to accelerate the cure of the polyisocyanates migrating from the clearcoat and the block polyisocyanates formulated in the base paint. Metal compounds, acidic phosphate esters and tertiary amine compounds may be used. Examples of these compounds include dibutyltin dilaurate, dioctyltin dilaurate, dibutyltin diacetate, tetrabutyldiacetoxy distanoxane, lead naphthenate, cobalt naphthenate, lead 2-ethylhexanoate, iron 2-ethylhexanoate, Phosphoric acid monobutyl ester, phosphoric acid dibutoxy ester, triethylenediamine and the like can be used.

The resin used for the clear coat paint comprises a resin containing an isocyanate-reactive active hydrogen. Specific examples include resins having a hydroxyl group or a carboxyl group. Acrylic resins, polyester resins, alkyd resins, polyurethane resins and the like containing active hydrogen used in the industry can be used. The active hydrogen in the active hydrogen-containing resin used for the clear coat paint is preferably a hydroxyl group. A hydroxyl group-containing acrylic resin is particularly suitable. For example, the active hydrogen content is 0.01 to 2% by weight, the acid value is 0 to 60 mgKOH / g, preferably 0 to 40 mgKOH / g, and the number average molecular weight is 500 to 100,000, preferably 5,000 to 300, Those in the range of 000 are used. For example, when the active hydrogen-containing resin has a hydroxyl group, the hydroxyl group content is 0.1 to 15% by weight, preferably 0.2 to 10% by weight.
As with the base coating material, a curing catalyst used in this field, for example, an organometallic compound, an acidic phosphate ester or a tertiary amine compound may be added.

  The coating method is not particularly limited, and an atomization method, a brush coating method, a roll method, a dipping method, a doctor blade method, and the like can be applied. Of these, atomization methods such as air spray, airless spray, and electrostatic coating are particularly preferable. The dry film thickness is preferably 5 to 100 μm, preferably 10 to 30 μm for the base coat layer, and 5 to 200 μm, preferably 20 to 80 μm for the clear coat layer.

  The base material to be painted may be various materials, for example, metal, resin (for example, plastic).

  EXAMPLES Next, although an Example demonstrates this invention further more concretely, this invention is not limited at all by the Example. The indication “%” means “% by weight” unless otherwise specified.

  The evaluation method evaluated each characteristic according to the following criteria.

<Measurement of content of HDI trimer and dimer in polyisocyanate>
It calculated | required by the outflow area percentage obtained by the measurement of a gel permeation chromatography (GPC). Peaks of molecular weight corresponding to HDI trimers or dimers were defined as trimers or dimers. The measurement conditions are as follows.
Measuring instrument: HLC-8120GPC manufactured by Tosoh Corporation
Column: One each of Shodex KF601, 602, 603,
Carrier: Tetrahydrofuran Detection method: Differential refractive index detector Pretreatment: Active NCO group was measured after reacting with methanol to inactivate it.
Molecular weight: PPG (polypropylene glycol) conversion

<Quantification of isocyanurate ring and iminooxadiazinedione ring in polyisocyanate>
Quantification was performed by observing a specific absorption region in the ring structure by NMR spectroscopy ( ¹³ C-NMR). The measuring instrument used was DPX400 manufactured by Bruker. It was carried out at a frequency of 100M Hz with about 50% of the samples in dry CDCl _3. As the frequency reference, 77.0 ppm of the solvent (CDCl ₃ ) was used. Data on chemical shifts of the compounds of interest were taken from literature (see Die Angewandte Makromolekulare Chemie 1986, 141, 173-183 and references cited therein) or obtained from measurements made on model substances. A result is represented by the molar ratio (mol%) of an isocyanurate ring and an iminooxadiazinedione ring.

<Permeability evaluation of polyisocyanate from clear coat to base coat>
Apply clear coat paint on the base coat painted on the polypropylene plate. This was heated and baked, and the obtained coating film was kept at room temperature, and then peeled off from the polypropylene plate. The infrared absorption spectrum (IR) absorption of the base coating film of polypropylene plate side was measured at the peak height of a wavelength after heating baking within 1 hour, IR of 2270 cm ^-1 for each 2930 cm ^-1 and 700 cm ^-1 Absorption peak height ratio (%) was calculated. Higher ratios indicate higher polyisocyanate penetration rates.
・ Wavelength 2930cm ^-1 (C-H stretching vibration absorption)
-Wavelength 2270 cm ⁻¹ (absorption of stretching vibration of isocyanate group —N═C═O)
・ Wavelength 700cm ^-1 (Aromatic ring absorption)
In infrared absorption spectrum (IR) absorption measurement, Nicolet 6700FT-IR manufactured by Thermo Fisher Scientific Co., Ltd. was used as a measuring instrument.
As a judgment, when the peak height ratio (%) is 3 times or more of the blank test without applying the clear coat (high permeability), “○”, and when the peak height ratio (%) is less than 3 times that of the blank (low permeability) Is shown as “x”.

<Appearance of coating film>
The appearance of the obtained coating film was visually evaluated according to the following criteria.
○: Excellent smoothness.
Δ: Smoothness is intermediate between “excellent” and “inferior”.
X: Smoothness is inferior.

<Waterproof interlayer adhesion>
A polypropylene coating plate after clear coat baking (see Example 1) obtained by applying a primer for polypropylene was immersed in warm water at 40 ° C. for 240 hours and then pulled up and dried at room temperature for 12 hours. Cut the coated film with a cutter knife so that it reaches the substrate, make 100 goby meshes of 2mm x 2mm size, apply adhesive cellophane tape to the coated surface, and peel off the tape rapidly at 20 ° C The coated surface was evaluated according to the following criteria.
○: 100 pieces of gobang eyes remain.
(Triangle | delta): 1-10 pieces of Goban eyes coating films have peeled.
X: 11 or more gobang eyes coatings are peeled off.

<Cohesive failure>
About the sample after said water-resistant interlayer adhesion evaluation, the destruction state of the gobang eyes coating film was evaluated according to the following reference | standard.
○: No destruction was observed inside the base coating film.
X: Breakage is clearly recognized inside the base coating film.

Prior to the examples and comparative examples, the following materials were prepared.
Acrylic polyol A (for base coat)
Bayhydrol A2427 (manufactured by Bayer MaterialScience)
Hydroxyl group-containing water-dispersed acrylic polyol, non-volatile content 42%, hydroxy group content 2% (non-volatile content conversion), viscosity 100 mPa · s / 23 ° C., pH 8-9, glass transition temperature 87 ° C., molecular weight Mw / Mn = 240000 / 22000

Melamine resin (for base coat)
Cymel 327 (manufactured by Cytec), methyl etherified melamine resin
Block isocyanate A (for base coat)
Death Module BL3475 (manufactured by Bayer MaterialScience)
Block isocyanate in which polyisocyanate consisting of HDI and IPDI (isophorone diisocyanate) is blocked with an active methylene compound, a solvent naphtha / butyl acetate solution with a nonvolatile content of 75%, a blocked isocyanate ratio of 8.2%, and a viscosity of 1000 mPa · s / 23 ° C.

Aluminum flake (for base coat)
EMR D5660 (Toyo Aluminum Co., Ltd.)

Thickener (for base coat)
Viscalex HV30 (Ciba Specialty Chemicals)

Surfactant A (for base coat)
A surfactant that is BYK347 (by Big Chemie),
A 1: 1 mixture of surfactant, Disperlon AQ320 (manufactured by Enomoto Kasei)

Cosolvent (for base coat)
Butyl glycol
Neutralizer (for base coat)
Dimethylethanolamine

Acrylic polyol B (for clear coat)
Desmophen A870 (manufactured by Bayer MaterialScience)
Butyl acetate solution with a nonvolatile content of 70%, solid hydroxyl group ratio 2.95%, glass transition temperature 27 ° C., viscosity 3500 mPa · s / 23 ° C., acid value 7.5 mgKOH / g, molecular weight Mw / Mn = 3400/1650

Surfactant B (for clear coat)
Surfactant which is BYK331 (by Big Chemie)
Leveling agent (for clear coat)
Modaflow (manufactured by Monsanto)
Solvent (for clear coat)
Methoxypropyl acetate / butyl acetate = 1/1 solution

Creation example 1
Synthesis of curing agent A:
A three-necked flask equipped with a stirrer was purged with nitrogen, 200 g of HDI was taken, and kept at 60 ° C. Next, a 50% tetrabutylphosphonium hydrogen difluoride solution was added dropwise as a catalyst in isopropanol / methanol (2: 1) until the NCO content of the crude reaction solution reached 43% over 4 hours. The reaction was then stopped by adding 103 mg dibutyl phosphate. The HDI monomer was removed by thin film distillation at 130 ° C./0.2 mbar to obtain a polyisocyanate having the following data.
Nonvolatile content: 100% (no solvent)
NCO content: 23.4%
Viscosity: 680 mPa · s / 23 ° C
Trimer content: 68% by weight
Dimer content: 4.0% by weight
HDI monomer content: 0.2% by weight
Molar ratio of isocyanurate ring to iminooxadiazinedione ring: 55 to 45

Creation example 2
Synthesis of curing agent B:
A. Preparation of catalyst solution 600 g of 2-ethylhexanol was added to 100 g of a 40% solution of N, N, N-trimethyl-N-benzylammonium hydroxide in methanol and stirred. The methanol was then removed with sufficient stirring at 30-40 ° C. with a water pump vacuum. 2-Ethylhexanol was added to the stock solution to adjust the catalyst concentration to about 0.5%.

B. Synthesis from HDI A three-necked flask equipped with a stirrer was purged with nitrogen, and 3200 g of HDI was taken and kept at 60 ° C. Then 32 g of the above catalyst solution was added dropwise over 30 minutes. Subsequently, stirring was continued for 2 hours while maintaining the temperature at 60 to 65 ° C. The NCO content of the crude product at that time was 38%. The reaction was then stopped by adding 0.32 g of a 25% solution of dibutyl phosphate in HDI. After cooling the liquid to room temperature, unreacted HDI was removed by thin layer distillation at 130 ° C./0.2 mbar to obtain a polyisocyanate having the following data.

Nonvolatile content: 100% (no solvent)
NCO group content: 23.0%
Viscosity: 1200 m Pa / s / 23 ° C
HDI monomer content: 0.2% by weight
Trimer content: 67% by weight
Dimer content: 0.5% by weight
Molar ratio of isocyanurate ring to iminooxadiazinedione ring: 95 to 5

Creation example 3
Synthesis of curing agent C:
A three-necked flask equipped with a stirrer was purged with nitrogen, and 1000 g of HDI was taken and kept at 60 ° C. To this was added 10 g of 1,3-butanediol as a cocatalyst and 3 g of tri-n-butylphosphine as a catalyst. The mixture was then kept at 60 ° C. for 4 hours to obtain a crude reaction mixture having an NCO content of 40%. To this, 2.8 g of p-toluenesulfonic acid methyl ester was added to stop the reaction. After thin-film distillation at 130 ° C. and a pressure of 0.15 mbar, a polyisocyanate having the following data was obtained.

Nonvolatile content: 100% (no solvent)
NCO content: 2 1.8%
Viscosity: 200 mPa · s / 2 3 ° C
Trimer content: 24% by weight
Dimer content: 38% by weight
HDI monomer content: 0.2% by weight
Molar ratio of isocyanurate ring to iminooxadiazinedione ring: 70:30

Creation example 4
Curing agents D to H were prepared as follows.
Preparation of curing agent D: Curing agent A and curing agent C were bundled at a weight ratio of 85:15.
Preparation of curing agent E: Curing agent A and curing agent C were bundled at a weight ratio of 75:25.
Preparation of curing agent F: Curing agent A and curing agent B were bundled at a weight ratio of 50:50.
Preparation of curing agent G: Curing agent A and curing agent B were bundled at a weight ratio of 25:75.
Preparation of curing agent H: Curing agent A and curing agent B were bundled at a weight ratio of 15:85.

Creation example 5
Synthesis of curing agent I (HDI allophanate-modified polyisocyanate):
A three-necked flask equipped with a stirrer was purged with nitrogen, and 302 g of HDI was taken and kept at 60 ° C. To this, 13.3 g of 1-butanol was added and stirred at 60 ° C. for 1 hour.
The temperature of the crude reaction mixture was then raised to 90 ° C. and 4.4% N, N, N-trimethyl-N-benzyl-ammonium hydroxide in 0.214 g 2-butanol was added to the 90 ° C. reaction mixture. The solution was added. When the reaction mixture reached an NCO content of 35%, the reaction was stopped by the addition of 0.21 g di- (2-ethylhexyl) phosphate. Unreacted monomer was removed by thin film evaporation at 130 ° C. and a pressure of 0.15 mbar to obtain a polyisocyanate having the following data.

Nonvolatile content: 100% (no solvent)
NCO content: 2 0.0%
Viscosity: 500 mPa · s / 23 ° C
HDI monomer content: 0.2% by weight

Creation example 6
Curing agent J (HDI biuret-modified polyisocyanate)
Death module N3200, non-volatile content manufactured by Bayer MaterialScience: 100% (no solvent)
NCO group content: 23.0%,
Viscosity: 2500 mPa · s / 23 ° C
HDI monomer content: 0.2% by weight

  Tables 1 and 2 show characteristic values such as NCO group content, viscosity, and composition of each curing agent.

Example 1
A base coat formulation BC-1 shown in Table 3 was prepared (non-volatile content: about 22%, Ford Cup No. 4 viscosity: 50-60 seconds). This was spray-coated on a polypropylene plate to a dry film thickness of 15 to 20 microns, allowed to stand at room temperature for 3 minutes, and further dried at 80 ° C. for 3 minutes to obtain a base coat film. Next, a clear coat formulation CC-1 shown in Table 4 was prepared (when CC-1 and a predetermined amount of curing agent were mixed, the nonvolatile content was about 50% and the Ford Cup No. 4 viscosity was 18 to 22 seconds). This was mixed with curing agent A so that the NCO / OH molar ratio was 0.9, and spray-coated on the base coat film obtained above to a dry film thickness of about 40 microns. After standing at room temperature for 3 minutes, it was baked at 100 ° C. for 20 minutes. After lowering to room temperature, the coating film was peeled off from the polypropylene plate.
The penetration ratio of the NCO group of the base coat film on the polypropylene plate surface side was measured by the infrared absorption spectrum (hereinafter abbreviated as IR) method described above, and the degree of penetration of the polyisocyanate from the clear coat to the base coat was measured. .
As a blank experiment, only the base coat film was applied, and the dried film was subjected to the same operation to measure the IR absorption ratio.
As a primer for polypropylene, a commercially available chlorinated polyolefin resin and an aqueous polyurethane resin (Disquacol U54, manufactured by Bayer MaterialScience) were mixed and prepared. This was coated on a polypropylene plate, allowed to stand at room temperature for 3 minutes, and then dried at 80 ° C. for 3 minutes. Thereafter, a base coat and a clear coat were sequentially applied and baked by the method as described above to obtain a desired coating film. This coating film was evaluated by appearance, waterproof interlayer adhesion, and cohesive failure.

Examples 2 to 10 and Comparative Examples 1 to 6
Experiments were conducted in accordance with Example 1 with the base coat blending, clear coat blending, curing agent usage, and NCO / OH molar ratios shown in Tables 5, 6 and 7 changed, and the blending amount of each component was changed.
The results are shown in IR absorption ratio, permeability evaluation, coating film appearance, water-resistant interlayer adhesion, and cohesive failure in Tables 5, 6 and 7.

＊：イソシアネレート基とイミノオキサジアジンジオン基の総量に対するイミノオキサジアジンジオン基のモル％比

*: Mole% ratio of iminooxadiazinedione groups to the total amount of isocyanate groups and iminooxadiazinedione groups

＊イソシアネレート基とイミノオキサジアジンジオン基の総量に対するイミノオキサジアジンジオン基のモル％比
−：未測定または未評価

* Mole% ratio of iminooxadiazinedione group to the total amount of isocyanatolate group and iminooxadiazinedione group-: not measured or not evaluated

＊：アクリルポリオールＡとブロックイソシアネートＡのＮＣＯ／ＯＨモル比は０．６

*: NCO / OH molar ratio of acrylic polyol A and blocked isocyanate A is 0.6.

＊：クリヤコート配合ＣＣ−１、９４．７部に対する硬化剤使用量

*: Curing agent use amount for clear coat formulation CC-1, 94.7 parts

＊：クリヤコート配合ＣＣ−１、９４．７部に対する硬化剤使用量
−：未測定または未評価

*: Use amount of curing agent for clear coat formulation CC-1, 94.7 parts-: not measured or not evaluated

＊：クリヤコート配合ＣＣ−１、９４．７部に対する硬化剤使用量
−：未測定または未評価

*: Use amount of curing agent for clear coat formulation CC-1, 94.7 parts-: not measured or not evaluated

  From the above results, it can be seen that Examples 1 to 8 show a high IR absorption ratio, excellent water interlayer adhesion, and cohesive failure in the NCO / OH molar ratio: 0.9 to 1.2. In particular, it was shown that if a curing agent containing 13 mol% or more of iminooxadiazinedione group was used, the effect of reducing the viscosity of the curing agent and improving the permeability of the coating film was great. Further, in Examples 9 and 10, it was confirmed that excellent coating performance was exhibited even in the formulation using blocked isocyanate in the base coat.

  On the other hand, in Comparative Examples 1 and 4, the viscosity exceeds 1000 mPa · s / 23 ° C., and the polyisocyanate having an iminooxadiazinedione group in the trimer is less than 13 mol%. Water-resistant interlayer adhesion cannot be obtained. In Comparative Examples 2 and 3, the penetrability is high, but since the trimer of the curing agent is less than 60% and the dimer exceeds 10%, sufficient waterproof interlayer adhesion cannot be obtained. In Comparative Examples 5 and 6, the polyisocyanate modification type is a curing agent different from that of the present invention, but both have low permeability and sufficient water interlayer adhesion cannot be obtained.

  The present invention is called a wet-on-wet system or a 2-coat 1-bake system for the purpose of shortening the coating process, saving energy, and reducing VOC (volatile organic compounds) in the field of coating steel sheets and plastics such as automobiles and home appliances. Can be used in painting methods. Since a specific polyisocyanate is used in the clear coat paint layer, it penetrates efficiently into the lower base coat paint layer and reacts with the isocyanate-reactive component in the base coat paint layer, greatly increasing the adhesive strength between the two layers. Improved and excellent film properties can be formed.

Claims (5)

A base coat paint containing an active hydrogen-containing resin is applied, and this is not cross-linked, and then a clear coat paint containing a curing agent and an active hydrogen-containing resin is applied on the base coat, and then the base coat and the clear coat are simultaneously cured by cross-linking. A two-coat one-bake method of coating, wherein the clear-coat paint is a two-component polyurethane paint having an NCO / H (active hydrogen) molar ratio of 0.9 to 2.0, A coating method in which the curing agent includes a polyisocyanate derived from 1,6-hexamethylene diisocyanate and satisfies all of the following conditions:
1) the curing agent is substantially free of diisocyanate monomer,
2) The viscosity of the curing agent is 1000 mPa · s / 23 ° C. or less.
3) The content of 1,6-hexamethylene diisocyanate trimer in the curing agent is 60% by weight or more, and 4) The content of 1,6-hexamethylene diisocyanate dimer in the curing agent is 10% by weight. %.   The method according to claim 1, wherein the curing agent for clear coat paint contains 13 mol% or more of iminooxadiazinedione ring of 1,6-hexamethylene diisocyanate.   The method according to claim 1 or 2, wherein the base paint contains an active hydrogen-containing resin and a blocked isocyanate.   The hardening | curing agent for clearcoat coating materials used in the method in any one of Claims 1-3.   The multilayer coating film which consists of a basecoat layer and a clearcoat layer obtained by the method in any one of Claims 1-3.