Method for varnishing manufactured articles made of wood or wood fibre

Description

[0001] The present invention relates to a method for varnishing manufactured articles made of wood or wood fibre.

[0002] Wood and wood substitutes such as, for example, MDF (Medium Density Fibreboard), LDF (Light Density Fibreboard), Trupan®, wood pulp, etc., are generally subjected to a varnishing process based on four main stages consisting of a pretreatment, the application of a primer, rubbing down and the final stage of varnishing. The term pretreatment is understood to refer to all the operations which precede the application of the primer to the manufactured article. These operations vary based on the type of material being treated, its general condition, the type of finishing which it is desired to produce and the level of quality which it is desired to achieve. As regards MDF, for example, the pretreatment consists essentially in rubbing down thoroughly; this operation is extremely important if a high-quality result is to be obtained.

[0003] The application of primer envisages at least two different passages to be carried out using liquid varnish, the aim of which is to stabilize the material for the purpose of the subsequent finishing stage. Between the various operations of applying the primer comes a stage of gentle rubbing down, the aim of which is to remove the surface imperfections of the manufactured article prior to the subsequent application.

[0004] Many varnishes can be used at this stage and are chosen depending on the type of application (by spraying, by dusting, with a roller, etc.) and depending on the type of material and/or passivation used. In general, these varnishes do not need conductivity properties, and the usual passivation temperature of the material is about 40°-80°C.

[0005] The actual varnishing stage determines the final appearance of the article; in the usual varnishing processes, varnishes diluted in solvents are used, these having various properties depending on the uses for which the manufactured article is intended or the type of application chosen.

[0006] The varnishing method described hereinabove requires the application of at least three coats (two applications of primer and the top coat, which is also diluted with solvent), and up to a maximum of seven coats. The large consumption of material substantially increases the production costs of the finished product, and at the same time gives rise to environmental problems which can mainly be brought down to the evaporation of large quantities of solvent and to the disposal of the product once it has become unusable.

[0007] The problem underlying the present invention is thus to provide a method for varnishing manufactured articles made of wood or wood fibre, which overcomes the abovementioned problems.

[0008] This problem is solved, according to the invention, by a method for varnishing a manufactured article made of wood or wood fibre, this method comprising a first stage of rubbing down the surface of the said manufactured article, characterized in that it comprises the following successive stages of:

a) preheating the said manufactured article uniformly;

b) applying one or more layers of a primer to the surface of the said manufactured article;

c) heating the said manufactured article;

d) varnishing the said manufactured article with a powdered varnish.

[0009] The varnishing method according to the invention requires a treatment of the manufactured article which differs from that proposed in conventional methods and which takes the form of simplification or replacement of some of the traditional operating stages.

[0010] The first condition for the method is the rubbing-down of the manufactured article, since its initial surface state is of considerable importance if a high-quality final result is to be obtained.

[0011] Once it has been rubbed down, the manufactured article is preheated. This stage, which is not present in traditional methods involving finishing with a liquid varnish, can be carried out using conventional kilns.

[0012] The final preheating temperature of the manufactured article depends on the type of material being considered and its thickness. In general, especially as regards materials composed of wood fibre, such as MDF, low thicknesses show greater surface hardness and this allows the preheating temperature of the material to be reduced. Preferably, the preheating temperature of the manufactured article is maintained in the region from 100 to 200°C.

[0013] An air temperature about 10-15°C greater than that required by the material used is maintained inside the kiln, in order to ensure that the manufactured article reaches the preset, temperature. The residence time in the kiln depends considerably on the dimensions of the manufactured article, on its composition and, lastly, on its degree of humidity; for the same material and thickness, a higher percentage of humidity generally requires a longer residence time in the kiln or, when this is not possible, a higher air temperature in the kiln.

[0014] For example, Trupan® boards 30 mm in thickness were subjected to temperatures of about 160°C for periods of between 10 and 20 minutes, giving excellent results. The same times, but with temperatures of about 120°C, were applied to Fantoni Plaxil 100 MDF boards, while longer times (30 min) but even lower temperatures (about 100°C) were applied to manufactured articles made of wood.

[0015] Preferably, the residence time in the preheating kiln is less than or equal to 40 minutes.

[0016] The application of primer after preheating the manufactured article is an essential stage and ensures a high-quality result for the powder-varnishing. The primer can be applied, for example, by immersion, by spraying or with a roller.

[0017] The powdered varnishes are applied using specific devices which charge the powder either positively (triboelectric guns, which charge the powder by direct friction) or negatively (corona-effect guns). The use of both devices necessarily requires the presence of a support which has specific electrical conductivity properties, this being a property which is not present in wood or in wood substitutes in the natural state.

[0018] The primer which is used in the method according to the invention must thus be stable at high temperatures (up to 250°C) and must have electrical conductivity properties. On applying it, the manufactured article becomes much less sensitive to temperature changes, since the primer inhibits, at least partially, any formation of cracks on its surface. In addition, the electrical conductivity properties acquired by the manufactured article treated with the primer described above allow powdered varnishes to be applied by conventional methods, thus simplifying the method as a whole.

[0019] Preferably, the primer is applied to the manufactured article while it is still hot, having a temperature of not more than 180°C; this approach helps the primer to penetrate into the fibres of the manufactured article, thus making it easier to stabilize its outer surface, and at the same time makes it possible to dispense with intermediate cooling apparatus. Preferably, the primer is applied to the manufactured article which is at a temperature ranging from 40 to 80°C.

[0020] Use of the primer PA1095 from the company ILVA Polimeri S.p.A. and of the primer Graphital Standard from the company Servimetal (France) have proved to be particularly effective.

[0021] If necessary it is possible to apply one or more additional layers of primer; this depends essentially on the degree of finishing required and on the type of varnishing powder which will be used thereafter.

[0022] In the case of a double application, the treatment envisages a first layer of primer on the manufactured article, which has been rubbed down beforehand, and, optionally, which is still at the temperature of the preceding preheating operation. If necessary, after primer has been applied, the manufactured article is again heated in order to bring it to temperature prior to the subsequent application. This intermediate heating stage is preferably carried out at temperatures ranging from 100 to 200°C.

[0023] In a specific embodiment of the present invention, the primer is applied prediluted so as to allow it to penetrate more deeply into the manufactured article, thus optimizing the operation. In the case of multiple layers of primer, this primer can be applied, independently from one layer to another, in pure or dilute form.

[0024] The dilution of the primer varies either according to the type of material which is to be treated or according to its method of application. Generally, the more compact the material, the lower the percentage of solvent which needs to be added to the base primer. For some materials, such as, for example, MDF Fantoni Plaxil 100, it can also be observed that by decreasing the thickness of the manufactured article, the required dilution is reduced.

[0025] In addition, the higher the temperature of the manufactured article when the primer is applied, the lower the percentage of solvent required. However, at a manufactured article temperature above 180°C, the results degrade due essentially to the increased rate of evaporation of the solvent with respect to the base primer.

[0026] The dilution also depends on the means used to apply the primer; for the same material at the same temperature, application with a roller generally requires a lower dilution than an application by spraying.

[0027] Once the stage of application of the primer has ended, the manufactured article has to undergo a heating stage. This operation must, however, be carried out by methods different to those applied in the preceding preheating stage, since the purpose of this new heat treatment is not only to dry the surface of the manufactured article, but also to prepare it for application of the film of powder and the subsequent polymerization of this powder.

[0028] Preferably, the manufactured article is treated at temperatures of between 130°C and 200°C, depending on the type of firing required by the particular powder used, and for periods of up to 40 min, while remaining both chemically and structurally stable. In addition, the manufactured article has to be freed of any final traces of moisture and of chemical products which might appear at the surface during the polymerization of the powder, thereby giving rise to bubbles, making the manufactured article fall short of the specificities required for varnishing (discontinuity of the surface of the film of powder, microbubbles, etc.).

[0029] The kiln temperature and the related residence time therein of the manufactured article depend closely on the type and dimensions of the manufactured article, and on the firing time of the varnishing powder chosen. However, the principle of reducing as much as possible the temperatures to which the manufactured article must be subjected remains valid, so as to minimize the stresses inflicted on the material.

[0030] In the experimental tests, carried out on MDF and Trupan® boards 20 and 30 mm in thickness, temperatures ranging between 150°C and 200°C for periods of between 10 and 30 minutes were applied, with excellent results. These boards were varnished using powders having polymerization temperatures of 150°C and 190°C; using powders with lower polymerization temperatures, the maximum heating temperature is 130°C.

[0031] At the end of the second heat treatment of the manufactured article, the next and final rubbing-down is carried out, if required. After rubbing down, the manufactured article is ready for the powder-varnishing and the final polymerization.

[0032] The choice of the type of varnishing powder automatically defines the polymerization cycle; it is possible to choose from among many categories of powders, not only based on the type of conditions which the manufactured article must face (internal, external, attacking atmospheres, etc.), but also based on the type of polymerization which it is desired to use.

[0033] In this respect, there are essentially two possible choices: polymerization in a kiln with air and polymerization by IR and UV irradiation. Preferably, however, the hot-air kiln route is followed, since it is considered to be more versatile, more robust with respect to any anomalies of the support and gives far fewer problems of calibration and setting-up of the plant in the field of application.

[0034] The powders which can be used for the varnishing stage are of conventional type; the optimum choice of the type of powder depends on many different factors which come down to the nature of the manufactured article.

[0035] Powders with moderate polymerization temperatures and times are preferably used, so as to reduce the thermal stresses to which the manufactured article is subjected. It should also be pointed out that these polymerization parameters influence not only the powder-varnishing stage, but also the preceding stages associated therewith. A low polymerization temperature of the powder makes it possible to reduce both the maximum heating temperature of the primer and the preheating temperature of the board.

[0036] Preferably, the powders which can be used in the method according to the invention have to form a film of high elasticity, high direct and indirect impact strength and high resistance to chemical and atmospheric agents.

[0037] Wherever necessary, it is possible to apply a second layer of powder in order to optimize, for example, the edge finishing.

[0038] In a particularly advantageous embodiment of the present invention, the heat treatments are carried out in hot-air kilns, thus minimizing any deformations which the manufactured article suffers by gradually increasing the temperature.

[0039] The present invention also provides a varnished manufactured article comprising a base made of wood or wood fibre, characterized in that the said base comprises, in the following order, one or more layers of a primer and a film of polymerized powdered varnish. Preferably, the primers used are those from the company IVLA Polimeri S.p.A. (code PA1095) and from the company Servimetal France (Graphital Standard), and the said film of varnish has an impact strength of at least 1.5 Nm (ECCA test T5), a drawing of at least 3 mm (ISO test 1520) and a bending of at least 3 mm (ISO test 1519).

[0040] The present invention also relates to the use of a primer and a powdered varnish for varnishing manufactured articles made of wood or wood fibre. A preferred embodiment of the present invention envisages the use of a primer from the company ILVA Polimeri S.p.A. (code PA1095) or from the company Servimetal France (Graphital Standard); this primer can be used in pure or dilute form.

[0041] A further aspect of this invention is the provision of a kit for varnishing manufactured articles made of wood or wood fibre, comprising a primer and a powdered varnish. In a preferred embodiment, the primer is from the company ILVA Polimeri S.p.A. (code PA1095) or from the company Servimetal France (Graphital Standard).

[0042] The powder-varnishing method according to the invention makes it possible, compared with conventional techniques using liquid varnishes, to reduce substantially the use of pollutants, such as varnishes and solvents. By this method, it is generally sufficient to apply a maximum of two coats of primer, which is optionally diluted, whereas in a traditional method, anywhere from a minimum of three applications of liquid varnishes (two coats of primer and the top coat, which is also diluted with solvent) up to a maximum of seven are needed.

[0043] The production plants used to carry out the method according to the invention also make it possible to recover up to 97% of the powder used, which can then be returned into the productive cycle. The remaining 3% is extracted using suitable absolute filters which purify the air by purely mechanical means, i.e. without the use of chemical substances of any type.

[0044] The method according to the invention makes it possible to reduce the production costs of the manufactured article substantially. The use of thermosetting powders makes it possible in most cases to dispense with the sanding operations required in conventional methods. This leads in itself to a reduction in the production costs, but is even more important for the fact that it is thus possible to carry out an entirely automated process without the need for restarting; the manufactured article can thus be coupled, in raw form, to a suitable transporter and released after completion of the polymerization, after a fixed time. In this way, the production is substantially rationalized.

[0045] The cost of the powdered varnish may be comparable to that of traditional liquid varnish, but the yield is higher as a result of the lower level of consumption and the efficiency of the recovery systems.

[0046] The method according to the invention also makes it possible to transfer onto supports made of wood or of wood fibres any type of finishing which is normally available on a metal support. In addition to broadening the range of possible finishings, by giving wooden manufactured articles the typical characteristics of a powder-varnishing, this makes it possible to enjoy the typical advantages of a powder-varnishing, such as, for example, increased resistance of the manufactured article to chemical and mechanical stresses, to wear and to atmospheric agents.

[0047] The preferred embodiments of the invention will now be described, by way of non-limiting examples, in order to understand it more clearly.

EXAMPLES

[0048] In each example, the primer was applied using a common manual gun with a bucket, with an inlet air pressure limited to 3.5 atm. and a spray nozzle 1.2 mm in diameter. In Examples 1-4, a primer from the company ILVA Polimeri S.p.A., code PA1095, was used, whereas in Examples 5 and 6 a primer from the company Servimétal France (Graphital Standard) was used. A solvent from the company ILVA Polimeri S.p.A., code TZ 50, was used to dilute the primer PA1095 (when necessary), while rain-water was used to dilute the Graphital Standard.

[0049] In each example, the powder was applied using the following apparatus:

Bollhoff
"Manual Powder System BPM 930 with Control Unit BPS 930"

[0050] The following control parameter values were used for all of the experimental tests:

Voltage:

70 KV

Current:

60 µA

Example 1

[0051] A squared and rubbed-down Trupan® board with dimensions of 30x300x300 (mm) was preheated to a temperature of 160°C for a period of 10 min. The primer prediluted to 15% was applied, by low-pressure spraying, to the board at a temperature of 150°C. A second layer of pure primer was applied (by low-pressure spraying) but only on the edges of the board, which at this point had reached a temperature of 100°C.

[0052] The product thus obtained was heated to a temperature of 185°C for a period of 20 min and, after returning to a temperature of 120°C, was varnished with a powdered varnish (epoxy polyester, Grigio Raggrinzato Metallizzato Pulverlac code 2220203501), which was polymerized at 180°C for a period of 22 min. The product thus obtained had good surface uniformity and excellent covering of the edges.

Example 2

[0053] A squared and rubbed-down Trupan® board with dimensions of 30x300x300 (mm) was preheated to a temperature of 160°C for a period of 10 min. The primer prediluted to 20% was applied, by low-pressure spraying, to the board at a temperature of 150°C. The product thus obtained was heated to 160°C for a period of 5 min and then a second layer of primer was applied (by low-pressure spraying) but only on the edges of the board, which at this point had reaped a temperature of 150°C.

[0054] The product thus obtained was heated to a temperature of 190°C for a period of 20 min, sanded down (light, 1000 particles/cm² grain) and, after returning to a temperature of 135°C, was then varnished with a powdered varnish (Nero Opaco Matt liscio Arsonsisi, code B140N00). The powder was finally polymerized at 185°C for a period of 22 min. The product thus obtained had good surface uniformity and excellent covering of the edges.

Example 3

[0055] A Fantoni Plaxil 100 MDF board with dimensions 10x300x300 (mm) was preheated to a temperature of 170°C for a period of 15 min. The pure primer was applied, with a roller, to the board at a temperature of 160°C.

[0056] The product thus obtained was heated to a temperature of 165°C for a period of 20 min, sanded down (light, 1000 pti/cm² grain) and, after returning to a temperature of 110°C, was then varnished with a powdered varnish (epoxy polyester, Grigio Raggrinzato Metallizzato, Pulverlac code 2220203501). The powder was finally polymerized at 160°C for a period of 35 min. The product thus obtained had good surface uniformity and excellent covering of the edges.

Example 4

[0057] A parallelepiped of beech with dimensions of 50x50x400 mm was preheated to a temperature of 110°C for a period of 30 min. The primer prediluted to 25% was applied, by low-pressure spraying, to the product at a temperature of 105°C. The product thus obtained was heated to 130°C for a period of 5 min and a second layer of primer diluted to 10% was then applied (by low-pressure spraying), the manufactured article at this point having reached a temperature of 120°C.

[0058] The product thus obtained was heated to a temperature of 190°C for a period of 20 min and, after returning to a temperature of 120°C, was varnished with a powdered varnish (epoxidic, Grigio Basalto Liscio Opaco, Europolveri, code 604.8.5611). The powder was finally polymerized at 185°C for a period of 22 min. The product thus obtained had good surface uniformity.

Example 5

[0059] A parallelepiped of Trupan® with dimensions of 130x85x30 mm was preheated to a temperature of 160°C for a period of 5 min. The prediluted primer (1:4 (vol) primer:rainwater) at a temperature of 70°C was applied, by partial immersion for 120 sec, to the manufactured article at a temperature of 150°C. The product thus obtained was heated to 160°C for a period of 10 min and was then sanded down lightly with a cloth of 800 particles/cm² grain.

[0060] The product, at a temperature of 120°C, was varnished a first time with a powdered varnish (Herberts, Rosso BT AT Senza Piombo, code 12.00.7115), heated in an kiln to 160°C for 3 min and then cooled to 130°C, at which temperature a second coat of powder was applied.

[0061] The powder was finally polymerized at 160°C for a period of 20 min. The product thus obtained had an excellent surface finish.

Example 6

[0062] A parallelepiped of Trupan® with dimensions of 130x85x30 mm was preheated to a temperature of 160°C for a period of 15 min. The prediluted primer (1:4 (vol) primer:rainwater) with a temperature of 70°C was applied, by partial immersion for 180 sec, to the product at a temperature of 150°C. The product thus obtained was heated to 160°C for a period of 15 min and then sanded down lightly with a cloth of 800 particles/cm² grain.

[0063] The product, at a temperature of 120°C, was varnished with a powdered varnish (Herberts, Rosso BT AT Senza Piombo, code 12.00.7115).

[0064] The powder was finally polymerized at 160°C for a period of 20 min. The product thus obtained had a good surface finish.

Claims

1. Method for varnishing a manufactured article made of wood or wood fibre, this method comprising a first stage of rubbing down the surface of the said manufactured article, characterized in that it comprises the following successive stages of:

a) preheating the said manufactured article uniformly;

b) applying one or more layers of a primer to the surface of the said manufactured article;

c) heating the said manufactured article;

d) varnishing the said manufactured article with a powdered varnish.

2. Method according to Claim 1, in which the said preheating is carried out to a temperature ranging from 100 to 200°C.

3. Method according to Claim 1 or 2, in which the said preheating of the manufactured article is carried out for a period less than or equal to 40 min.

4. Method according to any one of the preceding claims, in which the said primer is applied to the manufactured article which is at a temperature of less than or equal to 180°C.

5. Method according to Claim 4, in which the said primer is applied to the manufactured article which is at a temperature ranging from 40 to 80°C.

6. Method according to any one of the preceding claims, in which the said primer is chosen from PA1095 from the company ILVA Polimeri S.p.A. and Graphital Standard from the company Servimétal (France).

7. Method according to any one of the preceding claims, in which, in stage b), one or more said layers of pure or dilute primer are applied, independently of each other.

8. Method according to any one of the preceding claims, in which a single layer of primer is applied in stage b).

9. Method according to any one of Claims 1 to 7, in which two or more layers of primer are applied in stage b).

10. Method according to Claim 9, in which, in stage b), an intermediate heating stage is inserted between two applications of primer.

11. Method according to Claim 10, in which the said intermediate heating stage is carried out at a temperature ranging from 100 to 200°C.

12. Method according to any one of the preceding claims, in which the said stage of heating of the manufactured article takes place at a temperature ranging from 130°C to 200°C.

13. Method according to any one of the preceding claims, in which the said heating stage is carried out for a period less than or equal to 40 min.

14. Method according to any one of the preceding claims, in which the said stages of preheating and heating of the manufactured article are carried out in an kiln with air.

15. Manufactured article comprising a base made of wood or wood fibre, characterized in that the said base comprises, in the given order, one or more layers of a primer and a film of polymerized powdered varnish.

16. Manufactured article according to Claim 15, in which the said primer is chosen from PA1095 from the company ILVA Polimeri S.p.A. and Graphital Standard from the company Servimétal (France).

17. Manufactured article according to Claim 15 or 16, in which the said film of varnish has an impact strength of at least 1.5 Nm (ECCA test T5).

18. Manufactured article according to Claim 15, 16 or 17, in which the said film of varnish has a drawing of at least 3 mm (ISO test 1520).

19. Manufactured article according to one of Claims 15 to 18, in which the said film of varnish has a bending of at least 3 mm (ISO test 1519).

20. Use of a primer and of a powdered varnish for varnishing manufactured articles made of wood or wood fibre.

21. Use according to Claim 20, in which the said primer is chosen from PA1095 from the company ILVA Polimeri S.p.A. and Graphital Standard from the company Servimétal (France).

22. Use according to Claim 20 or 21, in which the said primer is used in pure form.

23. Use according to Claim 20 or 21, in which the said primer is used in dilute form.

24. Kit for varnishing manufactured articles made of wood or wood fibre, comprising a primer and a powdered varnish.

25. Kit according to Claim 24, in which the said primer is chosen from PA1095 from the company ILVA Polimeri S.p.A. and Graphital Standard from the company Servimétal (France).