A transparent wood coating composition that can impart flame retardancy simply by applying it

Abstract

To provide a transparent coating composition for wood which can impart good flame retardancy to a wood surface simply by applying it to the surface without impairing the texture of the wood (visibility of wood grain). [Solution] The clear coating composition for wood of the present invention is characterized by comprising: a primer coating composition containing a binder containing a specific composition of a precondensate of dicyandiamide, phosphoric acid, and formaldehyde and a predetermined amount of polyethyleneimine; a predetermined amount of a phosphorus compound with excellent carbonization properties as a flame retardant; and a top coating composition containing the binder and predetermined amounts of fibrous nanosilica and mica fine powder. The phosphorus compound with excellent carbonization properties can be at least one selected from phosphinic acid metal salts, melamine polyphosphate, and guanidine phosphate.

Description

[Technical Field]

[0001] The present invention relates to a transparent coating composition for wood that can impart excellent flame retardancy to a wood surface simply by applying it to the surface without impairing the texture of the wood (visibility of the wood grain). [Background technology]

[0002] The active use of wood in buildings is being promoted, with the enactment of the Act on the Promotion of Wood Use in Public Buildings and Buildings Contributing to the Realization of a Carbon-Free Society. The Building Standards Act generally imposes interior decoration restrictions on buildings such as homes, public facilities, and commercial buildings. These interior decoration restrictions are intended to delay the outbreak of fire and smoke by using flame-retardant materials for wall and ceiling finishes, and require the use of semi-noncombustible or flame-retardant materials.

[0003] In recent years, the use of domestic timber has been promoted in an effort to realize a decarbonized society, and the need for fire-retardant timber is growing. Traditionally, fire-retardant timber has been widely used, where timber is pressurized and impregnated with fire-retardant chemicals. However, this impregnation method is not only costly, but also increases construction time, and is difficult to apply to large pieces of timber such as CLT (Cross Laminated Timber, also known as cross-laminated timber). Furthermore, depending on the type of chemical, efflorescence and deliquescence can occur, and water resistance can also be an issue.

[0004] In response to this, inventions have been developed that impart flame retardancy to wood by coating the surface of wood without impregnating the wood with a flame retardant. For example, Patent Document 1 (Japanese Patent No. 6581123) discloses an invention for a coated substrate that has high fire resistance and allows the surface of the combustible substrate to be visible from the outside, by providing a coated substrate comprising an intermediate layer mainly composed of silica particles provided on a combustible substrate such as wood, and an alkali metal silicate (water glass) layer provided on the intermediate layer. This intermediate layer improves the uniformity of the surface properties of the combustible substrate such as wood, and improves the uniformity of the thickness of the alkali metal silicate layer provided on this intermediate layer, thereby allowing the fire resistance of the alkali metal silicate layer to be fully exhibited.

[0005] Furthermore, Patent Document 2 (JP 2023-176311 A) discloses an invention for a coating composition that contains a specific amino group-modified organopolysiloxane, one or more flame retardants selected from phosphorus-based, boron-based, magnesium-based, aluminum-based, nitrogen-based, antimony-based, and halogen-based compounds, and an inorganic filler, and that can impart flame retardancy to flammable substrates such as wood, and that produces coating films with good transparency and moisture resistance.

[0006] In addition, Patent Document 3 (JP 2020-122098 A) discloses an invention of a flame-retardant thermoplastic resin composition or flame-retardant thermosetting resin composition in which a flame retardant component consisting of a complex salt of phosphinic acid is blended with a thermoplastic resin or thermosetting resin such as polyurethane resin, vinyl chloride resin, polyester resin, or polyamide resin, and which has significantly improved water resistance, exhibits flame retardancy equal to or greater than that of halogen-based flame retardants, does not inhibit the colorability of the product, and combines decomposition resistance, hydrolysis resistance, and bleed-out resistance even under high temperature and high humidity conditions.

[0007] Furthermore, Patent Document 4 (Japanese Patent No. 7327879) discloses an invention relating to a flame retardant composition for thermosetting resins, which contains a flame retardant A made of a specific phosphorus compound and a flame retardant B made of magnesium hydroxide and / or aluminum hydroxide in a predetermined content ratio, and which is capable of easily forming a carbonized layer without melting or dripping during combustion.

[0008] Furthermore, Patent Document 5 (JP 2005-048035 A) discloses an invention of a flame-retardant resin composition which is made of a polyolefin resin composition such as low-density polyethylene, high-density polyethylene, polypropylene, or polybutene, containing a magnesium hydroxide-based flame retardant containing magnesium hydroxide that has been surface-treated with aqueous (poly)carbodiimide, and which has improved mechanical properties, water resistance, and acid resistance. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Patent No. 6581123 [Patent Document 2] Patent Publication No. 2023-176311 [Patent Document 3] Japanese Patent Application Publication No. 2020-122098 [Patent Document 4] Patent No. 7327879 [Patent Document 5] Japanese Patent Application Laid-Open No. 2005-048035 [Non-patent literature]

[0010] [Non-Patent Document 1] "Wood Preservation" Vol.13-4(1987)p.151-166 [Non-patent document 2] "Wood Preservation" Vol.14-1 (1988) p.8-17 Summary of the Invention [Problem to be solved by the invention]

[0011] According to the invention of the coated substrate disclosed in the above-mentioned Patent Document 1, a coated substrate is proposed in which silica particles are used as an intermediate layer in a primer to improve the fireproofing effect of alkali metal salts (water glass). However, alkali metal salts have poor conformability to deformation of the substrate, which causes problems such as cracking in the coating film and significant problems with water resistance. Furthermore, it is known that alkali metal salts react with wood, causing discoloration and whitening. To prevent these problems, a top coat layer is required, but the strong alkalinity of the water glass surface causes the adhesion of the top coat to decrease over time, which is an issue. Therefore, this composition is not suitable as a transparent coating composition for wood that can impart flame retardancy simply by application.

[0012] Furthermore, the amino-modified organopolysiloxane in the coating composition disclosed in Patent Document 2 places emphasis on water solubility and affinity with flame retardant components. Therefore, the flame retardant effect is small, so a large amount of flame retardant or inorganic filler must be added, which poses the problem of reduced transparency of the coating film. Furthermore, amino-modified silicones generally have the disadvantage of being prone to color changes, particularly yellowing, and are therefore not suitable as clear coating compositions for wood that can impart flame retardancy simply by application.

[0013] Furthermore, the flame retardant used in the invention of the flame-retardant thermosetting resin composition disclosed in Patent Document 3 is a non-halogen type metal phosphinate flame retardant. The flame-retardant thermosetting resin composition using this flame retardant has excellent long-term water resistance, and when added to paints, sealants, putties, adhesives, pressure-sensitive adhesives, etc., it can provide a water-resistant phosphinate flame retardant-blended resin composition that has excellent flame retardancy and shape retention after combustion tests. However, its main applications are plastics and urethane foam, and it is highly acidic, so it is not suitable as a clear coating composition for wood that can impart flame retardancy simply by application.

[0014] Furthermore, the flame retardant used in the invention of a flame retardant composition for thermosetting resins and a fiber-reinforced thermosetting resin disclosed in Patent Document 4 also produces a flame retardant composition for thermosetting resins and a fiber-reinforced thermosetting resin that have excellent flame retardant effects and shape retention after combustion tests. However, like the invention of the flame-retardant thermoplastic resin composition or flame-retardant thermosetting resin composition disclosed in Patent Document 3, the flame retardant is mainly used for plastics, urethane foam, etc., and is highly acidic and therefore not suitable as a transparent coating composition for wood that can impart flame retardancy simply by application.

[0015] Furthermore, according to the invention of a flame-retardant resin composition disclosed in Patent Document 5, a flame-retardant effect cannot be obtained unless a large amount of magnesium-based flame retardant is blended, so transparency cannot be expected, and the degradation of mechanical properties when a large amount of magnesium-based flame retardant is blended also poses an issue. Moreover, aqueous (poly)carbodiimide, which is used as a surface treatment agent for magnesium hydroxide, easily reacts and cures with resins containing carboxyl groups, and it is suggested that it be used together with polyolefin-based resins, but considering storage stability, it is not suitable as a transparent coating composition for wood that can impart flame retardancy simply by application.

[0016] The present invention has been made to solve the problems of the prior art as described above, and has been completed based on the discovery that by using a specific component as a binder in a clear coating composition for wood, and by ensuring that the flame retardant contained in this binder and the filler component for increasing the strength of the coating film do not mix with each other, i.e., by using an undercoat coating composition that contains the binder and flame retardant and is applied directly to the wood surface to form an undercoat coating film, and by using a topcoat coating composition that contains the binder and filler component and is applied to the surface of this undercoat coating film to form a topcoat coating film, the roles of the components contained in the binder can be well fulfilled, and good flame retardancy can be imparted to wood while maintaining transparency simply by application.

[0017] In other words, the object of the present invention is to provide a transparent coating composition for wood that can impart good flame retardancy without impairing the texture of the wood (visibility of the wood grain) by simply applying it to the wood surface, without impregnating the wood with a flame retardant treatment agent. [Means for solving the problem]

[0018] The clear coating composition for wood of the present invention, which can impart transparency and good flame retardancy simply by being applied to a wood surface, comprises: an undercoat coating composition containing a binder to which a precondensation product of dicyandiamide, phosphoric acid, and formaldehyde and polyethyleneimine have been added, and a phosphorus compound having excellent carbonization performance as a flame retardant; and a topcoat coating composition to which fibrous nanosilica and mica fine powder have been added to the binder, wherein the composition of the precondensation product of dicyandiamide, phosphoric acid, and formaldehyde is in a molar ratio of dicyandiamide:phosphoric acid:formaldehyde=1:(0.5-2):(1.5-3), the binder contains 0.5-10 parts by mass of polyethyleneimine per 100 parts by mass of the solids content of the precondensation product of dicyandiamide, phosphoric acid, and formaldehyde, and the undercoat coating composition contains 5-30 parts by mass of the phosphorus compound having excellent carbonization performance as a flame retardant per 100 parts by mass of the binder, The topcoat paint composition contains 10 to 60 parts by mass of the fibrous nanosilica and 5 to 30 parts by mass of the mica fine powder in terms of solid content, relative to 100 parts by mass of the binder. And, The phosphorus compound having excellent carbonization performance is at least one selected from the group consisting of phosphinic acid metal salts, melamine polyphosphate, and guanidine phosphate. It is characterized by:

[0019] According to the transparent coating composition for wood of the present invention, simply by applying it to the surface of wood, it is transparent and does not impair the texture of the wood (visibility of the wood grain), and the primer coating film based on the primer coating composition exhibits good flame retardancy.In addition, the topcoat coating film based on the topcoat coating composition does not contain any components in the binder other than the filler component that would deteriorate the properties of the topcoat coating film, and this filler component increases the strength of the topcoat coating film, suppressing the occurrence of cracks in the topcoat coating film during combustion, leading to further improved flame retardancy of the primer coating film.

[0020] As shown in the reaction formula below, the initial condensation product of dicyandiamide, phosphoric acid, and formaldehyde is thought to be a mixture of mainly methylolated guanylurea phosphate and methylolated dicyandiamide phosphate.

[0021] [Reaction scheme] JPEG0007727982000002.jpg108115

[0022] This initial condensation product of dicyandiamide, phosphoric acid, and formaldehyde is known to be a component that imparts flame retardancy to wood and the like, as shown in, for example, the above-mentioned Non-Patent Documents 1 and 2. However, this initial polymer alone is sticky, and upon aging it becomes a hydrophilic colloid and then a dispersed gel, and upon drying it becomes a water-insoluble resin, so it takes time to form a coating film.

[0023] Meanwhile, the binder in the clear coating composition for wood of the present invention is a precondensation product of dicyandiamide, phosphoric acid, and formaldehyde, and polyethyleneimine are blended together. This polyethyleneimine is a highly reactive water-soluble polymer with a branched structure containing primary, secondary, and tertiary amines polymerized from ethyleneimine, and acts as a crosslinking agent, reacting with the acid and cyanamide (dicyandiamide, melamine, etc.) in the precondensation product of dicyandiamide, phosphoric acid, and formaldehyde to produce an amide compound, forming a coating film-forming component that can form a coating film in a short period of time.

[0024] The binder in the clear coating composition for wood of the present invention, which has such a composition, not only serves as a coating film-forming component, but also provides flame retardancy through the formation of a foamed insulation layer based on the presence of a precondensate of dicyandiamide, phosphoric acid, and formaldehyde, but does not provide flame retardancy to a degree that would allow the wood to be directly applied to buildings. Therefore, the undercoat coating film formed by the undercoat coating composition in the clear coating composition for wood of the present invention not only has the flame retardancy of the binder itself, but also the flame retardancy of the new additive, a phosphorus compound, which has excellent insulating effect through carbonization of wood, resulting in a undercoat coating film with even better flame retardancy.

[0025] In addition, the topcoat film formed by the topcoat paint composition of the wood transparent coating composition of the present invention also has a certain degree of flame retardancy because the binder itself has excellent flame retardancy, and the filler components consisting of fibrous nanosilica and mica fine powder increase the strength of the topcoat film, thereby suppressing the occurrence of cracks in the topcoat film during combustion. Therefore, with the wood transparent coating composition of the present invention, the properties of the primer coating film formed by the primer coating composition and the topcoat coating film formed by the topcoat coating composition combine to produce a wood transparent coating composition with excellent transparency and flame retardancy.

[0026] In the transparent coating composition for wood of the present invention, the binder preferably contains 0.5 to 10 parts by mass of polyethyleneimine per 100 parts by mass of the solids of the precondensation product of dicyandiamide, phosphoric acid, and formaldehyde. Less than 0.5 parts by mass of polyethyleneimine per 100 parts by mass of the solids of the precondensation product of dicyandiamide, phosphoric acid, and formaldehyde is undesirable because the effect of adding polyethyleneimine is small and the stickiness of the coating film is not eliminated. Similarly, more than 10 parts by mass is undesirable because the storage stability of the transparent coating composition for wood of the present invention is reduced.

[0027] Furthermore, the primer coating composition in the transparent coating composition for wood of the present invention preferably contains 5 to 30 parts by mass of a phosphorus compound with excellent carbonization performance as a flame retardant per 100 parts by mass of binder, and the top coating composition preferably contains 10 to 60 parts by mass, calculated as solids, of fibrous nanosilica and 5 to 30 parts by mass of mica fine powder per 100 parts by mass of binder. If the content of the phosphorus compound with excellent carbonization performance in the primer coating composition is less than 5 parts by mass, calculated as solids, per 100 parts by mass of binder, the flame retardant effect is small, and similarly, if it exceeds 30 parts by mass, the transparency of the resulting primer coating film decreases and the strength of the primer coating film also decreases, which is undesirable.

[0028] Furthermore, if the content of fibrous nanosilica in the top coat paint composition is less than 10 parts by mass, calculated as solids, per 100 parts by mass of binder, the reinforcing effect of the composite heat insulating layer obtained during combustion will be small, and similarly, if it exceeds 60 parts by mass, the viscosity of the top coat paint composition will increase and storage stability will be reduced, which is undesirable.Furthermore, if the content of mica fine powder in the top coat paint composition is less than 5 parts by mass per 100 parts by mass of binder, the effect of suppressing cracking of the top coat paint film during combustion will be small, and similarly, if it exceeds 30 parts by mass, the viscosity of the top coat paint composition will increase and storage stability will be reduced, which is undesirable.

[0029] Furthermore, the molar ratio of the dicyandiamide, phosphoric acid, and formaldehyde precondensate in the binder of the wood transparent coating composition of the present invention is preferably in the range of dicyandiamide:phosphoric acid:formaldehyde = 1:(0.5-2):(1.5-3). If the phosphoric acid content is less than 0.5 moles per mole of dicyandiamide, paint stability and flame retardant effect are reduced, while if it exceeds 2 moles, the flame retardant effect is reduced and the coating surface becomes sticky, which is undesirable. Furthermore, if the formaldehyde content is less than 1.5 moles per mole of dicyandiamide, crystals may precipitate on the coating surface, reducing design properties, and if it exceeds 3 moles, the amount of unreacted formaldehyde increases, causing environmental problems, which is undesirable.

[0030] The phosphorus compound having excellent carbonization properties in the primer coating composition of the transparent coating composition for wood of the present invention is preferably at least one selected from phosphinic acid metal salts, melamine polyphosphate, and guanidine phosphate. These organic phosphorus flame retardants are known to not only suppress combustion reactions in the gas phase, but also to exert a flame-retardant effect by producing high-density phosphorus-containing char (charcoal) upon combustion, thereby providing a transparent coating composition for wood that can impart good flame retardancy to wood. [Effects of the Invention]

[0031] As described above, the transparent wood coating composition of the present invention can be obtained by simply applying it to the surface of wood, without impregnating the wood with a flame retardant treatment agent, and can provide a transparent wood coating composition that is transparent and can impart good flame retardancy without impairing the texture of the wood (visibility of the wood grain). [Brief explanation of the drawings]

[0032] [Figure 1] 1 is a photograph of the surface of a test specimen of Example 1. [Figure 2] 1 is a photograph of the surface of a test specimen of Comparative Example 4. DETAILED DESCRIPTION OF THE INVENTION

[0033] The transparent coating composition for wood of the present invention will be described in detail below using various examples and comparative examples. However, the various examples shown below are examples for embodying the technical idea of ​​the present invention, and are not intended to limit the present invention to those shown in these examples. The present invention can equally be applied to other embodiments included in the scope of the claims.

[0034] [Preparation of binder] First, we will explain the process for preparing a binder containing a precondensate of dicyandiamide, phosphoric acid, and formaldehyde, which is commonly used in the primer and topcoat compositions that make up the clear coating composition for wood of the present invention, and polyethyleneimine. First, 187 parts by weight (10.4 mol) of deionized water was added to a flask equipped with a reflux condenser, and stirring was initiated. Next, 201 parts by weight (1.7 mol) of 85% phosphoric acid was slowly added, followed by 136 parts by weight (1.6 mol) of dicyandiamide powder and 263 parts by weight (3.2 mol) of 37% formalin solution. After the addition was completed, the mixture was heated to 85°C and maintained at reflux for 2.5 hours. Next, the mixture was cooled to 60°C, and 8 parts by weight (0.13 mol) of boric acid powder was added. Stirring was continued for 10 minutes, and the mixture was then cooled to 40°C. A precondensate with a molar ratio of dicyandiamide:phosphoric acid:formaldehyde of approximately 1:1:2 was obtained, forming a clear solution with a pH of approximately 2.

[0035] This solution had a viscosity of 15 to 20 mPa·s / 23°C when the solid content was 50% by mass. 5 parts by mass of polyethyleneimine (Epomin SP-018 (trade name), manufactured by Nippon Shokubai Co., Ltd.) was mixed with 100 parts by mass of the solid content of this solution, and the mixture was thoroughly stirred and mixed at room temperature to cause a reaction, resulting in a binder that was used in common in the following examples and comparative examples.

[0036] [Example 1] To 100 parts by mass of the binder prepared as described above, 15 parts by mass of a phosphinic acid metal salt (EXOLIT OP1230 (trade name, manufactured by Clariant Chemicals)) as a phosphorus compound with excellent carbonization properties and flame retardant properties was added, followed by thorough stirring. This gave a primer coating composition for use in the wood clear coating composition of Example 1. Similarly, to 100 parts by mass of the binder, 33 parts by mass (solids equivalent) of a fibrous nanosilica dispersion (Snowtex UP (trade name, manufactured by Nissan Chemical Industries, Ltd., particle size 12 μm) and 17 parts by mass of mica powder (A-11 (trade name, manufactured by Yamaguchi Mica Co., Ltd., average particle size 3 μm) were added, followed by thorough stirring. This gave a top coating composition for use in the wood clear coating composition of Example 1. The primer coating composition and top coating composition prepared in this way were used as the wood clear coating composition of Example 1.

[0037] [Example 2] The transparent wood coating composition of Example 2 was prepared in the same manner as the transparent wood coating composition of Example 1, except that the content of the phosphinic acid metal salt in the primer coating composition of Example 1 was 10 parts by mass, and the content of the fibrous nanosilica dispersion in the topcoat coating composition was 25 parts by mass, converted to solids, and the content of the mica powder was 10 parts by mass.

[0038] [Example 3] The transparent wood coating composition of Example 3 was prepared in the same manner as the transparent wood coating composition of Example 1, except that the content of the phosphinic acid metal salt in the primer coating composition of Example 1 was 25 parts by mass, and the content of the fibrous nanosilica dispersion in the topcoat coating composition was 50 parts by mass, converted to solids, and the content of the mica powder was 25 parts by mass.

[0039] Example 4 The transparent wood coating composition of Example 4 was prepared in the same manner as the transparent wood coating composition of Example 1, except that the phosphinic acid metal salt in the primer coating composition of Example 1 was replaced with 15 parts by mass of melamine polyphosphate (PHOSMEL-200 (trade name, manufactured by Nissan Chemical Industries, Ltd.)).

[0040] [Example 5] The transparent wood coating composition of Example 5 was prepared in the same manner as the transparent wood coating composition of Example 1, except that the phosphinic acid metal salt in the primer coating composition of Example 1 was replaced with 15 parts by mass of guanidine phosphate (Nonnene W-200 (trade name, manufactured by Marubishi Yuka Kogyo Co., Ltd.)).

[0041] [Example 6] The transparent coating composition for wood of Example 6 was prepared in the same manner as the transparent coating composition for wood of Example 1, except that the content of the phosphinic acid metal salt in the primer coating composition of Example 1 was changed to 10 parts by mass, and 10 parts by mass of guanidine phosphate, as in Example 5, was further added.

[0042] Comparative Example 1 The transparent wood coating composition of Comparative Example 1 was prepared in the same manner as the transparent wood coating composition of Example 1, except that the content of the phosphinic acid metal salt in the undercoat coating composition of Example 1 was 2.5 parts by mass, and the content of the fibrous nanosilica dispersion in the topcoat coating composition was 5 parts by mass, converted into solids, and the content of the mica powder was 10 parts by mass. Phosphinic acid metal salts The content ratio is 2.5 parts by mass and Fibrous nanosilica in topcoat paint compositions is becoming less and less Dispersion The content ratio is 5 in solids equivalent Mass part and It's getting less.

[0043] [Comparative Example 2] The transparent wood coating composition of Comparative Example 2 was prepared in the same manner as the transparent wood coating composition of Example 1, except that the phosphinic acid metal salt in the primer coating composition of Example 1 was replaced with 15 parts by mass of aluminum hydroxide B103 (trade name, manufactured by Nippon Light Metal Co., Ltd., average particle size 7 μm).

[0044] Comparative Example 3 The transparent wood coating composition of Comparative Example 3 was prepared in the same manner as the transparent wood coating composition of Example 1, except that the content of the phosphinic acid metal salt in the primer coating composition of Example 1 was changed to 40 parts by mass. The primer coating composition in the transparent wood coating composition of Comparative Example 3 had a higher content of the phosphinic acid metal salt than the primer coating composition of Example 1. 25 parts by mass There are more and more.

[0045] Comparative Example 4 The transparent wood coating composition of Comparative Example 4 was prepared in the same manner as the transparent wood coating composition of Example 4, except that the content of the fibrous nanosilica dispersion in the topcoat coating composition of Example 4 was changed to 66 parts by mass in terms of solids content. The topcoat coating composition of Comparative Example 4 contains twice the content of the fibrous nanosilica dispersion as the topcoat coating composition of Example 4, which is 33% more by mass in terms of solids content.

[0046] Comparative Example 5 The clear wood coating composition of Comparative Example 5 was prepared in the same manner as the clear wood coating composition of Example 1, except that the primer coating composition of Example 1 further contained 15 parts by mass of aluminum hydroxide and the content of mica powder in the top coating composition was 3.3 parts by mass. Phosphinic acid metal salts In addition to containing 15 parts by mass of the above, the content of mica powder in the top coating composition is reduced to 3.3 parts by mass.

[0047] Comparative Example 6 The transparent wood coating composition of Comparative Example 6 was prepared in the same manner as the transparent wood coating composition of Example 1, except that the fibrous nanosilica dispersion in the topcoat coating composition of Example 1 was replaced with the same amount (33 parts by mass) of spherical nanosilica dispersion (Snowtex O (trade name, manufactured by Nissan Chemical Industries, Ltd., particle size 12 μm) and the content of mica powder was reduced to 3.3 parts by mass.

[0048] [Comparative Example 7] The transparent wood coating composition of Comparative Example 7 was prepared in the same manner as the transparent wood coating composition of Example 1, except that the content of mica powder in the topcoat coating composition of Example 1 was increased to 33 parts by mass.

[0049] [Paint Stability Test] To investigate the stability of each of the primer coating composition and top coating composition of the transparent coating composition for wood of Examples 1 to 6 and Comparative Examples 1 to 7 prepared as described above, each primer coating composition and top coating composition was left to stand for 30 minutes, observed visually, and tested for paint stability according to the following criteria: ×: Components separated or became cloudy ○: No abnormalities were observed The results are summarized in Table 1, along with the respective components.

[0050] [Painting Test] Using each of the wood clear coating compositions of Examples 1 to 6 and Comparative Examples 1 to 7, commercially available flame-retardant-treated cedar wood (knot-free) measuring 70 x 150 x 10 mm was prepared. First, the primer coating composition was applied by brush to a predetermined thickness under predetermined conditions and dried. Next, the topcoat coating composition was applied by brush to a predetermined thickness under predetermined conditions and dried to obtain each test specimen. The appearance of each test specimen was then visually observed, and the paint appearance was evaluated according to the following criteria: ◯: The coating film was transparent, the wood grain was visible, and no efflorescence or deliquescence was observed. ×: The wood grain was not visible, or efflorescence or deliquescence was observed. The results, along with the respective components, are summarized in Table 1. A surface photograph of the test specimen of Example 1 is shown in Figure 1, and a surface photograph of the test specimen of Comparative Example 4 is shown in Figure 2.

[0051] [Flame retardancy test] Test specimens prepared using the wood transparent coating compositions of Examples 1 to 6 and Comparative Examples 1 to 7 were subjected to a heat release test using a cone calorimeter. The cone calorimeter is a device that calculates the heat release rate, total heat release, and other combustion parameters of a material from the amount of oxygen consumed during combustion, based on the principle that the amount of heat generated during combustion is 13.1 MJ per 1 kg of oxygen, regardless of the type of organic material. Here, a radiant heat intensity of 50 kW / m 2 A cone calorimeter test (ISO-5660-1) was carried out to evaluate the flame retardancy according to the following criteria. 〇: Calorific value after 10 minutes of heating is 8 (MJ / m 2 ) or less × : The calorific value after 10 minutes of heating is 8 (MJ / m 2 ) ××: Test failed due to coating foaming and contact with the ignition device The results are summarized in Table 1 along with the respective components.

[0052] [Table 1]

[0053] The results shown in Table 1 reveal the following: Comparing the results of Examples 1 to 3 and 6 and Comparative Examples 1 and 3, the content of the phosphorus compound with excellent flame retardancy in the primer coating composition was found to be poor when the content of the phosphinic acid metal salt, which is a phosphorus compound with excellent flame retardancy, in the primer coating composition was as low as 2.5 parts by mass per 100 parts by mass of binder (Comparative Example 1), whereas the content of the phosphinic acid metal salt was as high as 40 parts by mass (Comparative Example 3), resulting in poor paint stability, paint film appearance, and flame retardancy. Considering that the corresponding results, where the content of the phosphinic acid metal salt was 10 to 25 parts by mass (Examples 1 to 3), were good in all respects of paint stability, paint film appearance, and flame retardancy, it can be seen that the content of the phosphorus compound with excellent flame retardancy in the primer coating composition is preferably 5 to 30 parts by mass per 100 parts by mass of binder.

[0054] In Comparative Examples 2 and 5, which did not contain a phosphorus compound with excellent flame retardancy in the primer coating composition but contained aluminum hydroxide instead, the paint stability and coating appearance were good, but the flame retardancy was poor. Considering that good results were also obtained in the cases where melamine polyphosphate (Example 4) or guanidine phosphate (Example 5) was used instead of the metal phosphinate salt, and even when a metal phosphinate salt and guanidine phosphate were used in combination (Example 6), it is recognized that it is an essential constituent requirement for at least the primer coating composition to contain at least one of the above-mentioned phosphorus compounds with excellent flame retardancy in addition to the binder.

[0055] Furthermore, when the topcoat paint composition contained 5 parts by weight (solids content) of fibrous nanosilica dispersion per 100 parts by weight of binder in the topcoat paint composition binder (Comparative Example 1), the primer paint composition contained a low 2.5% by weight of phosphinic acid metal salt, a phosphorus compound with excellent carbonization properties as a flame retardant, per 100 parts by weight of binder. This resulted in good paint stability and coating appearance, but poor heat resistance. However, when the topcoat paint composition contained 25 to 50 parts by weight (solids content) per 100 parts by weight of binder (Examples 1 to 6), good results were obtained in paint stability, coating appearance, and heat resistance. Furthermore, when the topcoat paint composition contained a high content of fibrous nanosilica dispersion per 100 parts by weight of binder, as high as 66% by weight (solids content) (Comparative Example 4), heat resistance was good, but paint stability and coating appearance were poor. Therefore, the optimum content of the fibrous nanosilica dispersion to be contained in the topcoat paint composition binder is thought to be 10 to 60 parts by mass, calculated as solid content, per 100 parts by mass of the binder.

[0056] Furthermore, comparing the results of Examples 1 and 2 and Comparative Examples 5 and 7 with respect to the mica powder content, when the mica powder content in the top coat paint composition is as low as 3.3 parts by mass per 100 parts by mass of binder (Comparative Example 5), the paint stability and paint film appearance are good, but when it is as high as 33 parts by mass (Comparative Example 7), all of the paint stability, paint film appearance, and flame retardancy are poor. Considering this result and the fact that good results were obtained in all of the paint stability, paint film appearance, and flame retardancy when the mica powder content was 17 parts by mass (Example 1) and 10 parts by mass (Example 2), it is clear that the mica powder content in the top coat paint composition should preferably be 5 to 30 parts by mass per 100 parts by mass of binder.

[0057] Furthermore, when a spherical silica dispersion (Comparative Example 6) was used in the topcoat paint composition together with the binder and mica powder, the paint film foamed and came into contact with the ignition device of the cone calorimeter, resulting in a failed test. However, when a fibrous nanosilica dispersion was used (Examples 1-6, Comparative Examples 1-5 and 7), it was possible to measure the calorific value using the cone calorimeter test, even when the flame retardancy was poor (Comparative Examples 1-3, 5 and 7), or when the paint stability and paint film appearance were poor (Comparative Examples 3, 5 and 7). From these results, it is recognized that the inclusion of a fibrous nanosilica dispersion in the topcoat paint composition together with the binder and mica powder is an essential constituent requirement.

[0058] From the above, it has been confirmed that by using the transparent wood coating composition of the present invention, which comprises a binder to which a specific composition of an initial condensate of dicyandiamide, phosphoric acid, and formaldehyde and a predetermined amount of polyethyleneimine have been added, an undercoat coating composition containing a predetermined amount of a phosphorus compound with excellent carbonization performance as a flame retardant, and a topcoat coating composition to which predetermined amounts of fibrous nanosilica and mica fine powder have been added, it is possible to form a coating film that is transparent and can impart good flame retardancy without impairing the texture of the wood (visibility of the wood grain) simply by applying it.

Claims

[Claim 1] A transparent coating composition for wood that can impart transparency and good flame retardancy to a wood surface simply by applying it to the wood surface, an undercoat coating composition comprising a binder to which a precondensation product of dicyandiamide, phosphoric acid, and formaldehyde and polyethyleneimine have been added, and a phosphorus compound having excellent carbonization performance as a flame retardant; and a topcoat coating composition to which fibrous nanosilica and mica fine powder have been added to the binder, wherein the composition of the precondensation product of dicyandiamide, phosphoric acid, and formaldehyde is in the range of dicyandiamide:phosphoric acid:formaldehyde=1:(0.5 to 2):(1.5 to 3) in terms of molar ratio, and the binder contains 0.5 to 10 parts by mass of polyethyleneimine per 100 parts by mass of the solid content of the precondensation product of dicyandiamide, phosphoric acid, and formaldehyde, and the undercoat coating composition contains 5 to 30 parts by mass of the phosphorus compound having excellent carbonization performance as a flame retardant per 100 parts by mass of the binder, The topcoat paint composition contains 10 to 60 parts by mass of the fibrous nanosilica and 5 to 30 parts by mass of the mica fine powder, calculated as solid content, per 100 parts by mass of the binder, and the phosphorus compound with excellent carbonization performance is at least one selected from phosphinic acid metal salts, melamine polyphosphate, and guanidine phosphate.

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