EP2450109A1

EP2450109A1 - Powder coating

Info

Publication number: EP2450109A1
Application number: EP10190536A
Authority: EP
Inventors: Peter Øvre
Original assignee: Grumble & Marker Industries Inc
Current assignee: Grumble & Marker Industries Inc
Priority date: 2010-11-09
Filing date: 2010-11-09
Publication date: 2012-05-09

Abstract

The invention provides a method of applying a powder coating to a surface of objects (10), in particular heat sensitive objects. The method uses a coating powder which when heated to a first temperature (T1) assumes tacky properties, and when heated to a second temperature (T2) higher than the first temperature (T1) will cure. The method comprises applying heat to the surface of the object (10) so as to heat a first surface layer of the object to the first temperature (T1); applying the coating powder to the heated surface of the object (10); and applying heat to the surface of the object (10) so as to heat a second surface layer of the object (10) and the applied coating powder to the second temperature (T2). The method is particularly useful for powder coating objects (10) comprising heat sensitive materials such as wood, medium-density fibre board, gypsum and plastics.

Description

FIELD OF THE INVENTION

This invention relates to powder coating of surfaces. Wet paint coating is applied using e.g. a brush or a spraying device and needs to dry or cure after having been applied. Powder coating implies application of a layer of powder that is heated to a temperature where the powder melts or cures and flows to form a coating. When the coating has cooled it is finished and the thus coated product can be handled. The main difference between a conventional liquid or wet paint and a powder coating is that the powder coating does not require a solvent to keep the binder and filler parts in a liquid suspension form.

BACKGROUND OF THE INVENTION

Powder coating is a type of coating that is applied as a free-flowing, dry powder. The main difference between a conventional liquid paint and a powder coating is that the powder coating does not require a solvent to keep the binder and filler parts in a liquid suspension form. The coating is typically applied electrostatically and is then cured under heat to allow it to flow and form a "skin". It may be used to create a hard finish that is tougher than conventional liquid paint. Traditionally, powder coating is mainly used for coating metals. The powder may be a thermoplastic or a thermosetting polymer or a mixture thereof. When heated, the thermosetting components of a coating powder will undergo an irreversible chemical curing process which will make the resulting coating stable and durable in use.
Electrical and thermal conductivity are often related so that materials having high electrical conductivity will also have high thermal conductivity and vice versa.
Metals generally have high electrical and thermal conductivity and are insensitive to heat. In processes for powder coating surfaces of metal objects the objects are heated to temperatures that are mainly determined based on the properties of the coating powder, in particular the temperature that is needed for melting or curing the coating powder to flow and form a skin on the coated surface. Such temperatures are usually well below temperatures that will affect metals to be coated.
Powder coating of objects of non-conductive materials and heat sensitive materials requires processes and conditions that are different from those for coating electrically conducting materials such as metals or other heat insensitive materials. Heat sensitive materials such as wood and medium-density fibreboard (MDF) used e.g. for furniture, or constructional elements in the building industry, such as gypsum boards and window frames are heat sensitive and may degrade, decompose and degas if subjected to temperatures traditionally used in processes for powder coating metals, and water in the material may be caused to evaporate and dry-out the objects. Such effects cause changes in the material and may cause dimensional changes and cracks in the surface, which is highly undesired when used e.g. in furniture. All such materials have a low electrical conductivity as compared to metals which are considered conductors.
For many years the industry has tried to find ways to apply powder coating to non-conductive materials, due to the many advantages powder coating has to offer compared to other methods of painting and coating. The main challenge has been to evenly attach the coating powder to a non-conductive item, achieving a smooth and evenly distributed surface on all sides of the production object. While several methods of pre-heating have been tried out, none were able to provide a fully suitable environment for the application of powder, without causing structural damage of the production object by applying excessive heat or using adhesives.

SUMMARY OF THE INVENTION

The invention provides a method of applying a powder coating to a surface of an object whereby the above-mentioned problems are avoided or at least reduced to a tolerable minimum. The method comprises the following

a) providing a coating powder which when heated to a first temperature assumes tacky properties, and when heated to a second temperature higher than the first temperature will cure;
b) applying heat to the surface of the object so as to heat a first surface layer of the object to the first temperature;
c) applying the coating powder to the heated surface of the object; and
d) applying heat to the surface of the object so as to heat a second surface layer of the object and the applied coating powder to the second temperature.

The application of heat to the surface of the object so as to heat a first surface layer of the object to the first temperature serves to prepare the surface of the object for receiving the coating powder. The most critical part of the production process is to manage the temperature of the objects. Therefore only a surface layer of the object is heated whereby it is avoided that internal material portions below the surface layer are affected.
The coating powder can be applied by conventional and suitable means, but a powder coating station in the form of a spray booth is preferred where the coating powder will be sprayed as a mist towards the object to be coated. When the coating powder comes into contact with the heated surface of the object the powder will be heated to the first temperature where it becomes tacky and will adhere to the heated surface.
The method of the invention is particularly suited for powder coating objects of electrically or thermally non-conducting materials and heat sensitive materials.
Mineral materials such as stone, bricks, tiles with or without a layer of glazing may advantageously be coated using the method of the invention. Heating only a surface layer and not the entire object saves energy.
The method of the invention is environmentally safe and production friendly. It utilizes a number of previously known technologies in a new combination and with new parameters to provide an advantageous production method. In order to apply the method of the invention with satisfying results, precision is essential.
Due to an extremely short production cycle of a few minutes and the complete lack of environmental hazards, this new method is suitable for integration into an existing production line of furniture, gypsum ceiling tiles, prefabricated windows, etc.
The invention is useful for powder coating all types of wood, MDF, pressed chipboard, gypsum board, most plastics materials, most varieties of paper and cardboard, most varieties of carton, laminated materials, concrete, ceramics, mineral wool, fibre cement, hard foam materials, stone etc. It is also possible to treat delicate and brittle materials, like gypsum, carton, paper and expensive woods, but also laminated materials. This is due to the fact that the heat treatment in pre-heating and during the solidification process is kept at the lowest possible temperatures where only the surface of the material is being heated whereby the material's structure is preserved.
Many of the advantages are known from applications with conductive materials, but have until now not been utilized with non-conductive materials. The reason why powder coating has become increasingly popular is that it is one of the very best technologies to provide maximum protection for the surface of materials. Powder coating provides a smooth and homogenous surface which is easy to clean and withstands many environmental hazards, like some types of asset, wear and tear and UV radiation, amongst others. Also the edges are evenly covered resulting in a solidly protected production object. When applied correctly the powder coat seals the object which has been treated, which means that even materials which for example are not resistant to water become waterproof. The coat melts or flows into the surface and creates a durable and strong layer of protection. Powder coating is therefore highly suitable for items which have to withstand climatic challenges of outdoor applications. The invention allows materials which until now had to be painted with traditional methods to be ready for shipment within few minutes, rather than hours or even days. A typical production time is 7 minutes. The method does not need environmentally hazardous solvents and is not poisonous in any way. The life cycle of the surface protection for items treated with the method of the invention is many years and the manufacturer is able to grant a warranty of 10 years or more.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows schematically a powder coating system for use when coating objects in accordance with the invention;
Figure 2A is a longitudinal view of the interior of a pre-heating station of figure 1;
Figure 2B is a transversal section through the pre-heating station of figure 2A;
Figure 3A is a longitudinal view of the interior of a powder coating station of figure 1;
Figure 3B is a transversal section through the powder coating station of figure 3B;
Figure 4A is a longitudinal view of the interior of a solidification station of figure 1; and
Figure 4B is a transversal section through the solidification station of figure 4A.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

In figure 1 is illustrated schematically a production process flow where objects 10 to be powder coated are suspended from a transport rail 20 by means of rail hangers 21 and transported through a pre-heating station 30, a powder coating station 40 and a solidification station 50. The objects 10 can be any objects, but the invention is particularly useful for powder coating heat sensitive objects such as furniture or furniture components and building constructional elements such as window frames consisting of or comprising wood or wood fibres, such as medium density fibre (MDF) board, or gypsum ceiling tiles or boards covered by a cardboard sheet.
Figures 2A and 2B show a pre-heating station 30 with heating elements 31 on both sides of the objects 10 for preheating the objects. The heating elements 31 preferably generate infrared (IR) radiation directed towards the objects 10, and ventilation fans 32 may be used to circulate the air to assist in obtaining an even application of the heat to the objects to avoid cold spots. The IR-generating heating elements are preferably LPG gas powered catalysers or electrically powered heating elements. IR reflectors 33 are arranged in the pre-heating station 30 to give a desired uniform IR irradiation of the objects 10.
In figure 1 is also seen an example of the surface temperature of the objects as a function of time in each of the three stations 30, 40 and 50. The heating power and the dwell time in the pre-heating station 30 are adjusted to result in a heating of the objects to a first temperature T1 so as to prepare them for receiving a layer of coating powder. In case the objects are heat sensitive only a surface layer of the objects is heated.
When the first temperature T1 has been obtained the objects are transported to the powder coating station 40 that forms an enclosure as seen in figures 3A and 3B. Coating powder is supplied to powder nozzles 41 through corresponding powder hoses 42 and sprayed onto the objects 10. The enclosure of the powder coating station 40 and the transport rail 20 and rail hanger 21 are kept at the same electrical potential such as ground potential, and there is thus substantially no difference in the electrical potential of the objects 10 and that of the enclosure of the powder coating station 40.
The coating powder is supplied from the powder hoses through powder guns 43 to the powder nozzles 41. The powder guns 43 are of the type called a tribo gun, which charges the powder by (triboelectric) friction, where the powder picks up a positive charge while rubbing along the wall of a Teflon tube inside the barrel of the gun. These charged powder particles are attracted electrostatically to the electrically grounded objects. Care should be taken to ensure that the coating powder is used in the precisely suitable quantity in order to avoid a faulty finish on the final production object, and in order to ensure a uniform distribution of the coating powder the powder guns 43 can be moved e.g. up and down when applying the powder.
The coating powder has a composition that ensures that the powder will assume tacky properties when it is heated to the first temperature T1 of the objects. When the electrically charged coating powder contacts the heated surface of the objects, the powder will assume the temperature T1 of the object and become tacky and adhere to the surface of the objects.
The powder coating station may have one or more suction filters 44 that withhold coating powder and prevent powder from escaping to the environment, and in a clean production environment excess powder can be collected and re-used.
When the objects 10 have obtained a desired layer of coating powder they are transported from the powder coating station to the solidification station 50.
Figures 4A and 4B show the solidification station 50 with heating elements 31 on both sides of the objects 10 for heating the objects 10 received from the powder coating station 40. The heating elements 51 preferably generate infrared (IR) radiation directed towards the objects 10, and ventilation fans 52 may be used to circulate the air to assist in obtaining an even application of the heat to the objects. The IR-generating heating elements 51 are preferably LPG gas powered catalysers or electrically powered heating elements. IR reflectors 53 are arranged in the solidification station 50 to give a uniform IR irradiation of the objects 10.
At the solidification station heat is applied to the surface of the objects 10 so as to heat a second surface layer of the object and the applied coating powder to a second temperature T2 which is higher than the first temperature T1 causing the applied coating powder to cure and solidify in a melting and/or curing process form a sealing skin on the surface of the objects. In this process timing it is important, and also to apply the exact amount of heat for the exact period of time required for completing the process to ensure a solid fusion with the surface without causing heat-induced structural damage to the objects. In case the objects are heat sensitive only a surface layer of the objects is heated. The system operates in such way that just enough heat is applied to solidify the powder and, and to avoid damage to the production object. The course of heating at the solidification station as a function of time is illustrated in figure 1.
The second temperature T2 is maintained for a period of time sufficient to ensure that the coating powder will form a skin on the surface of the objects, and also allowing thermosetting components of the powder composition to cure. This period is called the curing time. In case the objects are heat sensitive this period should be short in order to minimise heating of the interior of the objects below the heated surface layer, and the second temperature T2 should be low.
The first and second temperatures T1 and T2 and the curing time are parameters of the composition of the coating powder. When coating heat sensitive objects a coating powder having low first and second temperatures T1 and T2 and also a short curing time should be chosen whereby heating the objects to excessive temperatures for extended periods of time can be avoided and heating of the objects can be limited to a surface layer for a limited period of time. An elevated temperature may result in a shorter curing time and vice versa, and the combination of time and temperature should be chosen to ensure both minimal heat-induced changes in the objects and also a good resulting coating.
Due to the fact that only a very thin surface layer of the objects is heated, the temperature of entire object quickly drops to temperatures which allow almost immediate handling and packaging. The solidification process fully hardens the surface and no special attention needs to be given to the handling of the final powder coated product.
Experimental results are shown schematically in the charts in the upper half of figure 1 where the surface temperature of MDF objects are shown as a function of time during powder coating using the method of the invention. Objects enter the pre-heating station 30 at room temperature and after about 2 minutes the surface has reached a temperature T1 of about 70 degrees C at which the actual powder becomes tacky. From the pre-heating station 30 the objects are transported to the powder coating station 40 where the surface temperature drops to about 40 degrees C during about 4 minutes. At the solidification station the surface temperature rises to about 140 degrees C during about three quarters of a minute and is kept at about this value for about 2½ to 3 minutes whereafter the objects leave the solidification station and are allowed to cool. After cooling the newly powder coated objects are ready for handling.

Claims

A method of applying a powder coating to a surface of an object (10), the method comprising
- providing a coating powder which when heated to a first temperature (T1) assumes tacky properties, and when heated to a second temperature (T2) higher than the first temperature will cure;

- applying heat to the surface of the object (10) so as to heat a first surface layer of the object to the first temperature (T1);

- applying the coating powder to the heated surface of the object (10); and

- applying heat to the surface of the object (10) so as to heat a second surface layer of the object (10) and the applied coating powder to the second temperature (T2).
A method according to claim 1 further comprising
- bringing the object (10) into an enclosure (40) and applying a first electrical potential to the enclosure (40) and to the object (10);

- applying an electrical charge to the coating powder so as to confer to the coating powder a second electrical potential different from the first electrical potential; and

- applying the thus charged coating powder to the object (10) by spraying the charged coating powder.
A method according to claim 2 further comprising
- using an infrared heat source (31, 51) for applying the heat.
A method according to claim 1 wherein the object (10) comprises a heat sensitive material.