NO141434B - PROCEDURE FOR PREPARING COVERINGS ON WOODEN MATERIALS BY APPLYING A PRIMER AND A PHOTOSENSINIZED POLYESTER PULP - Google Patents

Description

Polyester covering materials, which contain unsaturated

polyesters as the most important component have gained great importance in technology.

By adding suitable accelerators, as is well known, the curing initiated by peroxides can be carried out at normal room temperature.

Of course, the curing agents and accelerators used must

be coordinated with each other.

It is also known to harden polyester-based coatings and fillers by irradiation with ultraviolet light. In this curing reaction, the peroxide curing agents and cobalt accelerators are eliminated.

In their place, a photochemically active sensitizer is used.

Under the influence of UV rays, this splits off radicals,

whereby the polymerization of the polyester layer is initiated. The

UV rays used here are generally produced by high-pressure or low-pressure lamps or by superactinic fluorescent tubes.

In contrast to the peroxide-induced curing of unsaturated polyester covers, the curing process here preferably begins from the outside in, i.e. it begins in the outer layer and proceeds in the direction of the tree. The inner layer that adheres to the wood material eventually hardens.

The curing of the polyester layer by UV rays has many advantages. However, it exhibits a number of shortcomings which have so far prevented a broad technical application. Thus, the adhesion of UV-cured polyester covers to wooden materials and veneers is worse than the peroxide-cured covers.

A further shortcoming consists in the fact that darkly stained woods and wood materials, such as soapstone, mahogany, walnut, macore or other coarse-pored woods turn grey.

The gray coloring is caused by the polymerization reaction proceeding from the outer surface inward in connection with the known volume contraction of photosensitized, polymerizing polyester masses, as these polyester masses are extracted from the wood pores during the curing process.

It has surprisingly been found that these disadvantages do not occur with a two-step process for producing polyester coatings by applying a primer layer and a coating layer and curing by means of ultraviolet light. The method is characterized by applying a primer layer which, as an activator, contains one or more metals from the fourth period of the periodic table, in salt form, but not polymerizable monomers, in an amount of 10-30 g/m , after which this primer layer is applied a photosensitized polyester layer which as a catalyst contains at least one peroxide compound of formula I, II or III:

in

where R denotes a methyl group or a branched alkyl-

group, R 2 a methyl group or a phenyl group, and R<3> is an H atom, a branched alkyl group, an aralkyl group with alkyl substituents in the aliphatic chain or an acyl group of an aliphatic, branched carboxylic acid residue.

The compounds that are contained in the combination according to the invention of primer layer and coating layer as activators (i.e. metal) and catalysts (i.e. peroxide) are matched to each other.

The activators in the sense of the present invention include the metals from the fourth period in the periodic table, in particular the side group metals with atomic numbers 21 - 30 in the form of their salts soluble in water or organic solvents. Preferably those metal compounds are suitable in which the metal is present in one of its higher valences. Particularly preferred is vanadium. A suitable vanadium salt is vanadyl para-toluenesulfonate. In the primer, the metal compound is present in such an amount that, calculated as metal, it makes up 0.02% - 0.06% of the primer material.

Such peroxides with the formulas I, II and III, which are soluble in the polyester coating mass, are advantageously used. Suitable is e.g. methyl isobutyl ketone peroxide, 2,5-dimethylhexane-2,5-dihyroperoxide, tert-butyl hydroperoxide, cumene hydroperoxide, di-tertiary-butyl hydroperoxide, tert-butyl cumyl peroxide, dicumyl peroxide, tert-butyl peroxyisononanate. These compounds are listed in the following table and assigned to the formulas I, II and III:

A particularly advantageous combination between metal compounds from the fourth period and the peroxides of formulas I, II or III are vanadium salts together with cumene hydroperoxide. This combination exerts a surprisingly strong effect, which not only prevents the wood material's pores from turning grey, but beyond this, the bond between wood and varnish is significantly improved.

Thus additions of only 0.02% - 0.06%, calculated as metal, are sufficient in the primer, and from 1.5 to 3.5% peroxide calculated for the coating compound.

It is also surprising that the effect only occurs when the metal compounds are in the primer layer, which is in direct contact with the wood material. There is no effect when the peroxide is in the primer layer and the metal compounds in the polyester layer laid on top.

The technique of applying the peroxide as a primer is mentioned in Norwegian patent no. 119,890, p. 5, penultimate paragraph. It is called the "active-basic method", and the underlying philosophy is that one should avoid

allowing the two reaction components to come into contact with each other before they are placed on the substrate to be coated. This philosophy is also used when using two-component spray guns, see Erich Karstens: "Lackrohstoff-Tabellen", 4. Auflage (1967)

p. 345. It was nevertheless not known to apply the metal-containing component first, even if one could also thereby provide an "active ground".

Namely, from the preceding Norwegian application no. 169,744

known to produce a kind of "active primer" by applying a zinc salt. However, the zinc ions here should not catalyze any cross-linking reaction, but themselves - with their two valences - cross-link an acid group-containing copolymer that was applied on top".

Not only the photosensitized polyester mass in the last step, but also the primer can be based on unsaturated polyesters, which, however, must here be free of copolymerizable monomers, such as styrene. However, it can also be based on other synthetic resins or nitrocellulose. It is usually not pigmented in the case of darkly stained types of wood. However, they can also contain soot and/or soluble dyes, whereby the primer simultaneously exerts a mordant or coloring effect. With this precaution, a work step is saved, and the special use of stain is eliminated, which means another major technical advance.

The primer is applied by pouring, spraying or rolling on the material to be coated. The primer layer is dried in the air. It can be dried more quickly at higher tape temperatures, up to 100°C. The amount of application is small, it is approx. 20 g/m 2 and should preferably not exceed 30 g/m 2. As a polyester coating compound applied to the primer layer, not only polyester varnish is suitable, but also any other colorless polyester coating material, such as e.g. colorless casting powders or rolling materials. The cover is produced by rolling, spraying or pouring. As a rule, 400 - 600 g of polyester material is applied at this stage per m 2, preferably approx. 400 g/m 2. Curing in the system takes place with ultraviolet light. The optimal wavelength for radiation is between 350 and 380 nanometers.

The parts given in the following examples are parts by weight. To carry out the examples, a polyester varnish mixture of 55 parts of an unsaturated polyester with an acid number of 45, which is normally obtained from two molecules of maleic acid, one molecule of phthalic acid and 3.2 molecules of 1, is used in all the cases described below. 2-propylene glycol, and stabilized by the addition of 0.02 parts hydroquinone, 45 parts styrene, 0.1 parts paraffin. The lacquer mixture has further been activated with 1.5 parts of a UV sensitizer (benzoin/ethyl ether). This varnish mixture is referred to in the examples briefly as "polyester varnish".

Example 1.

On a wooden board with a macoré or mahogany veneer, which is stained dark brown, a primer of the following composition is rolled on with a lacquer roller machine:

30 parts of a 10% highly viscous nitrocellulose solvent

ning in butyl acetate,

3 parts of a common ketone resin (dissolved in a cyclohexanone/methylcyclohexanone mixture), 4 parts of vanadium-para-toluenesulfonate, dissolved in an isopropanol/

xylene mixture with a metal content of

1%, and

63 parts butyl acetate.

After a drying time of 1 minute at 40°C, the polyester varnish is applied by rolling, spraying or pouring. The varnish was mixed with 4 parts cumene hydroperoxide in the form of a 70% mixture in a mixture of alcohol, ketone and cumene. 400 g/m 2 polyester varnish was applied. Curing by UV light took place in two stages. After an evaporation time of approx. 2 minutes are irradiated in the first step, followed by 2-3 minutes with a low-pressure lamp or a fluorescent tube. It is then irradiated for 15 - 30 seconds with a mercury high-pressure lamp.

A hardened polyester coating is produced, which is free from graying and exhibits excellent adhesion. The quality after grinding and polishing is excellent.

Example 2.

Work is carried out analogously to example 1. In the primer, instead of the vanadium compound, copper sulphate is used as an activator. The result is the same as in example 1.

Example 2a

Work is carried out analogously to example 1. In the primer, however, instead of the vanadium compound, iron III chloride is used as an activator.

Example 3.

Work is carried out as in example 1, only that instead of the vanadium compound, calcium octoate is used in the primer, and the polyester varnish is mixed with di-tertiary-butyl hydroperoxide. Here, too, the result after curing with UV light is a polyester layer which shows no tendency to grey, and which after grinding and polishing shows excellent quality.

Example 4.

Work is carried out analogously to example 1. Instead of the vanadium compound ice, CrCl3 is used in the primer and, instead of cumene hydroperoxide, tertiary butyl perisononanate is used in the polyester varnish. After curing with UV light, a polyester layer is obtained which has no tendency to grey, and which, after grinding and polishing, shows a surface of excellent quality with excellent adhesion to the wood material.

Example 4a.

Work is carried out analogously to example 1. Instead of the vanadium compound, titanium III chloride is used in the primer and instead of cumene hydroperoxide, tertiary butyl perisononanate is used in the polyester varnish. After curing by UV light, a polyester layer is obtained which has no tendency to grey, and which, after grinding and polishing, shows a surface of excellent quality and excellent adhesion to the wood material.

Example 5.

A primer with the following composition is rolled onto a walnut veneered chipboard with a lacquer roller machine: 40 parts of a 70% polyester resin solution in butyl acetate (the polyester resin is an unsaturated polyester with an acid number of 45, which is normally produced from two molecules of maleic acid , one molecule of phthalic acid and 3.2 molecules of 1,2-propylene glycol), 4 parts of vanadium-para-toluenesulfonate solution in isopropanol/xylene mixture with a metal content of 1%,

4 parts soot,

23 parts ethylene glycol monomethyl ether

23 parts ethyl alcohol, and

6 parts neozapon dye brown (mixture of equal parts of the dyes Color Index Solvent Black 27 and do. Yellow 82).

After a drying time of 2 minutes at 50°C, work is continued as in example 1. The polyester glue created after curing with UV light is free of dulling tendencies and shows ef-

ter grinding and polishing an excellent quality and good adhesion to the veneer sheet.

Claims (3)

1. Method for coating wooden materials by applying a primer and a photosensitized coating compound based on an unsaturated polyester and curing by exposure to ultraviolet light rays, characterized by applying an amount of 10 - 30 g/m in a continuous layer a primer which as an activator contains one or more metals from the fourth period of the periodic table in salt form, and which is free of polymerizable monomers, and over the primer layer applies a photosensitized polyester coating compound, which as a catalyst contains at least one peroxide compound of formula I, II or III where R denotes a methyl group or a branched alkyl group, R 2 a methyl group or a phenyl group, and R<3> is an H atom, a branched alkyl group, an aralkyl group with alkyl substituents in the aliphatic chain or an acyl group of an aliphatic, branched carboxylic acid residue . 2. Method according to claim 1, characterized in that salts of metals in a higher valence are used as activators in the primer. 3. Method according to claims 1 and 2, characterized by using salts in an amount of 0.02 - 0.06% by weight, calculated as metal, in the primer and 1.5 - 3.5% by weight of peroxide in the polyester coating compound.