WO2006000036A1

WO2006000036A1 - Plywood signage and powder coat composition

Info

Publication number: WO2006000036A1
Application number: PCT/AU2005/000920
Authority: WO
Inventors: David Murray James
Original assignee: Orica Australia Pty. Limited
Priority date: 2004-06-25
Filing date: 2005-06-24
Publication date: 2006-01-05

Abstract

Outdoor signage comprising powder coated MDF or plywood. The powder coating preferably contains a degassing agent such as Powdermate 542DG or Benzoin.

Description

PLYWOOD SIGNAGE AND POWDER COAT COMPOSITION

Field of the Invention This invention relates to plywood signage intended for outdoor use.

Background of the Invention Signs for use outdoors have a number of key characteristics. Outdoor signs should be durable and have good weathering characteristics in order to last despite repeated exposure to heat, cold, rain, hail, snow, sun and UV light. Ideally the signs will function despite mechanical damage such as that caused by thrown rocks and other objects.

Outdoor signage also needs to have good optical characteristics. The signs should be able to present information clearly at a distance. The substrate should provide or accept a suitable surface on which desired information can be presented. The substrate should be scratch resistant insofar as it affects the presentation of information thereon.

Signage can have a number of other requirements depending on the intended use. Traffic and street signs are used in large numbers in western countries. The substrate used in the signs and the sign blanks need to be capable of being worked and machine shaped and fastened to a support structures such as a pole or side of a building.

Wooden signs have been used for many years. Wood can be easily worked and shaped, either in a factory or on site by the use of saw. It may be used bare or unprotected when the substrate has satisfactory weathering characteristics or when the local environment provides suitable protection. More commonly, the wooden substrate will be protected from the elements by the application of paint. However, the paint degrades over time and the sign may require regular repainting to clearly present the information and to protect the substrate.

Nowadays, the cost of natural timber has significantly increased and instead signs may be made from manufactured or engineered wood products. A large number of different types of engineered wood products exist and may be utilised in the present invention. Reconstituted wood substrate (RWS) is substrate produced from wood particles, fibres, flakes or chips such as hardboard, medium density fibreboard (MDF)₅ wafer board, flake board, chip board and particle board. RWS is produced by combining the particles, fibres, flakes or chips with a binder and compressing into a sheet. Plywood is another engineered product and is a laminate formed from joining relatively thin layers of veneer together, with the grain of adjacent layers at right angles. Plywood may have a core of RWS.

Manufactured woods can provide significant cost advantages over nature timbers and also have the advantage of consistency, in that different batches of the same grade product should meet the same quality standards. However most manufactured woods, including plywood, have a poor resistance to water and make it unadvisable to use such products as a substrate for outdoor signage. Even when protected with a paint layer, a manufactured wood substrate can quickly fail once exposed to the weather. One of the problems with the use of manufactured woods is that once water gets in under the protective paint coating, the cellulose fibres can absorb the water and expand. This expansion can degrade the mechanical properties of the substrate and also lead to further water damage as cracks in the protective coating are widened.

Nowadays, traffic and street name signs are typically provided on a metal substrate. The metal, typically steel, is shaped and powder coated to provide a sign blank. Information is then painted onto the blank. Metal substrates are suitable for powder coating as the powder is applied using electrodeposition techniques and the mechanical properties of the substrate metal are not affected during the heat curing of the powder coating. However, metal substrates can significantly expand and contract during day and night cycles and can result in damage to the paint layer. Rust and electrolytic reactions with metal supports can also cause problems with metal substrates. Furthermore, unlike wooden signs, it can be difficult to rework the shape of a sheet metal sign on site.

The modern alternatives to the metal street sign include the use of rigid plastic sheet materials. Brief Description of the Invention In an embodiment of the invention there is provided outdoor signage comprising powder coated MDF or plywood.

In another embodiment of the invention there is provided a signage comprising a plywood or MDF core and a weather protective and water resistant powder coating on the core on which information may be presented.

In another embodiment of the invention there is provided the use of powder coated MDF or plywood for outdoor signage, such as shop, street or traffic signs.

In a further embodiment of the invention there is provided a powder coating formulation as described below in the examples for use on plywood or MDF to form outdoor signage.

Detailed Description of the Invention The invention is predicated on the surprising discovery that it is possible to powder coat plywood and MDF and provide a coating capable of forming an effective water resistant barrier. The coating has properties which enables the use of the powder coated substrate in outdoor signage. The use of MDF or plywood provides an alternative low cost substrate which may be machine shaped and also shaped by hand on site.

This result is surprising as there has been a general belief that it is not feasible to use MDF in outdoor signage, such as street signs, as MDF easily absorbs water, even when coated. Once the MDF absorbs water it will crack and split and dimensionally change shape. Small defects in a powder coating on the MDF or plywood can quickly lead to the failure of the signage once it has been exposed to the elements.

Unfortunately, it is commonplace for there to be defects in the protective powder coating when applied to plywood or MDF. In general it is difficult to successfully powder coat materials which are temperature sensitive, such as manufactured wood substrates. Traditional powder coating compositions fuse and cure the powder coating at temperatures over 180°C. Such temperatures are unacceptable for woods, particular manufactured woods, as it can damage the strength of the end product.

The curing temperatures can also result in distortion or internal splitting (checking) of the sheets and outgassing resulting in surface defects such as pin holing and poor adhesion. These defects generally are as a result of the release of moisture and other volatiles from the substrate. A further problem typically associated with the powder coating is poor coverage at the edges of the sheets. These deficiencies may not cause as significant problems when the powder coated plywood or MDF is intended for use within a controlled environment, such as within a household, but the deficiencies limit the use of the same powder coated plywood or MDF as outdoor signage.

The powder coating process involves applying a polymer powder to the substrate, typically by electrostatic coating techniques, and then fusing and temperature and / or UV curing the powder so that the particles melt, flow and fuse together and transform into a smooth, typically high gloss, coating. It is an environmentally friendly method of applying a coating as a solvent is not required and the overspray particles that are not bonded to a substrate can be collected and re-used in the next powder coating application. It is envisaged that a broad range of powder coating compositions and application methods could be used.

There are a number of problems with powder coating wood. The electrostatic coating techniques act by charging the particles and require the substrate to be coated to hold an opposite charge. Whilst this works well with metals, it is more difficult to form and hold the required charge on wooden substrates, particularly at the edges. Obtaining satisfactory coverage at and near the edges is important for signage, particular when coating pre-shaped wooden blanks to avoid the need for additional coating steps. This is less important when coating large sheets for subsequent shaping as there would need to be an additional coating step to protect the formed edges.

It can be helpful to improve charge retention by pre-heating the substrate sheets to a temperature over 8O⁰C. This increases the amount of water at or near the surface of the sheets and permits a charge to be more easily held. Other techniques that may be used include applying a brief burst of water or steam to the sheets or applying a conductive coating to the substrate. The latter approach has some disadvantages, not the least being requiring an additional step and the waste, solvents and drying time associated therewith.

Another method for improving charge retention is to incorporate sufficient amounts of electrically conductive materials within the plywood or MDF composite. The inclusion of metal fibres, the use of metal powders, inorganic salts such as sodium chloride, carbon black and other conductive materials as additives to the composite may significantly enhance charge retention.

The powder coating can be applied as a primer coat which is subsequently coated with top powder layer or, may include pigments for use as a single layer coating. As it is necessary to provide a water resistant, and more preferably water proof coating, then it is preferred to have multiple layers of the powder coating. This also can provide for smooth surface finish on the signage. The use of a primer powder coating layer may significantly reduce the need to pre- sand the uncoated MDF or plywood and provide a good painted surface finish. When the primer layer includes significant amounts of a texture additive, such as Bentone, then the primer layer can be sanded smooth before the application of the top coat.

The powder coating may be applied to the plywood and MDF using techniques adapted from those used to coat other reconstituted cellulose containing substrates, such as particle board, as known to the art. Two useful references are Volume 1 of Powder Coatings: The Technology, Formulation and Application of Powder Coatings by David M Howell, John Wiley & Sons Ltd, London, 2000 and The Technology of Powder Coatings by S.T.Harris, Portcullis Press Ltd, Surrey UK, 1976. MDF and plywood will be damaged by the use of high temperatures or by the application of moderate heat over a prolonged period.

As earlier mentioned, the moisture resistance of the coatings will be damaged due to vapour emissions from the substrate. Thus, it is preferred to apply the powder coating composition using electrostatic techniques and in a way which minimises unnecessary heating of the substrate sheets during the curing of the coating. Preferred methods involve the use of UV curable powder coatings, powder coatings which cure at low temperatures, such as those including low temperature curing agents or a combination of both UV and low temperature curing coatings.

The powder coating process may also include other techniques used to avoid the application of excessive heat to the substrate. It can be useful to pre-heat the substrate for a short time at a moderate heat, for example, by 2 to 10 minutes (preferably 5 minute) at 60 to 80°C (preferably 70°C). The use of a burst of steam shortly before coating may also preheat the substrate and improve charge retention. Low temperature curing techniques can be enhanced by the use of localised heating with IR lamps which reduce the heat exposure of the underlying substrate. Hg containing lamps have been found to be effective for UV curing.

The powder coating utilised may be any suitable commercially available powder coating composition. Typically, the powder coating will be based on polyester, epoxy, hybrid blends of polyester / epoxy, polyurethane and other suitable resins. Preferably, it will be UV curable and / or low temperature curable. Polyester resin systems are preferred for external use as epoxy resins can exhibit significant colour and structure degradation with long term exposure to sunlight. However, epoxy resins may still be used for outdoor signage in locations which are not generally subjected to direct sunlight.

The coatings are generally prepared by adding the required amounts of the raw materials into a premixer in which the ingredients are mechanically mixed, usually with a metal blade, to form a homogeneous mixture. This premix material passes through an extruding process. In this process the mixture is processed under heat (usually between 8O⁰C and 14O⁰C) and compounded using mechanical shear. This causes the powder coating composition to melt and act like a semi-liquid, and allows the ingredients to be intimately mixed into the powder coating composition. After leaving the extruder the material is cooled, generally on a chiller belt. The cooled mixture is then milled (ground) to the required particle size distribution for good application. A standard particle size distribution ranges from 2 to 200 microns, preferably 10 to 150 microns and typically around a medium size of 50 - 60 microns. The resin may contain colour pigments, extender pigments, cross-linkers and other additives. Examples of pigments and fillers include metal oxides, such as titanium oxide, iron oxide, zinc oxide and the like, metal hydroxides, metal powders, sulphides, sulphates, carbonates, silicates such as aluminium silicate, carbon black, talc, kaolins, barytes, iron blues, lead blues, organic reds, organic maroons and the like. The pigment (including extender pigment) can comprise up to 40% of the formulation depending on colour.

A slip-enhancing additive may be included to improve coating wear characteristics such as that described in US 5,925,698. Powder coating compositions may contain other coating modifiers such as polytetrafluoroethylene modified waxes, polyethylene waxes, polypropylene waxes, polyamide waxes, organosilicones and blends of the above. Polytetrafluoroethylenes (PTFE) may be used as a slip-enhancer / coating modifier. The inclusion of significant amounts (0.2% by weight or greater) of Telfon or other PTFEs such as Dyneon TF 1641 or Ceraflour 969 may also provide other benefits, not limited to scratch resistance. It is thought the inclusion of 1 or 2% by weight of PTFE may improve the bonding between the substrate and powder coating formulation.

Vertical (hanging substrate) or horizontal coating systems could be used in the coating process. Each system has advantages. Horizontal powder coating systems, such as that described in US 2003/0211252, may be of particular use with longer lengths of the substrate than could be reasonably attached to a hanging conveying system and should permit the powder coating of the main contact face and edge faces. Hanging systems allow the substrate to be entirely coated in a single pass with multiple electrostatic guns that apply the powder to all sides. Alternatively, electrostatic guns could be used to spray the powder on at least one face of a suspended substrate. Horizontal systems can be used to provide a wholly coated substrate in sequential powder coating steps. Horizontal systems can also allow the use of alternative powder delivery techniques such as fluidised beds or allowing powder to fall onto the substrate, by using for example a vibratory hopper. When coating pre-formed signage all surfaces of the substrate should be coated in order to improve overall water resistance and thereby reducing or preventing contact between the underlying plywood or MDF core and environmental water. Alternatively, the edge faces may be separately coated with a water proof sealant. This may also be done to the edges of a coated wood substrate, after it has been cut to a desired size.

The plywood or MDF may be coated as sheets or formed signage blanks or intermediate products. Whilst it is envisaged that the invention would be applied to provide street and traffic signs, it may also be used in the production of other types of outdoor signage, including irregularly shaped signs and signs having non-planar surfaces.

The size and dimensions of plywood or MDF sheets can vary widely. For example, the length of each sheet may be from 100 mm to 3500 mm. The width may vary from 30 mm to 2000 mm. The thickness may be, for example, from 2 mm to 50 mm.

The coating powder is typically applied to achieve a cured thickness of 0.04 to 0.6 mm, and preferably less than 0.1 mm. The powder is typically applied at a thickness of from 0.08 mm to 0.13 mm. The substrate could be coated with multiple layers to increase the thickness of the coating.

The powder coating can be applied by any suitable electrostatic technique. The two major techniques used are the corona electrostatic technique and the triboelectrostatic technique. According to the corona electrostatic technique the powder particles are given an electric charge as they come out of the end of a powder coating corona gun by electrodes located at the end of the gun tube. The electrodes are powered by a power-pack which can generate up to 100,000 V (100 KV). The usual working range for voltage is 50 to 100 KV. The powder is sprayed (powder is carried in a stream of air) at the earthed composite panel. The charge on the powder particles allows the powder particles to adhere to the substrate. After the powder coating is sprayed, a baking process is required to melt and chemically react and cross-link (creating a thermoset paint finish) the powder coating resin and the cross-linker. The triboelectrostatic technique involves a tribogun which also works by charging the powder particles towards an earthed panel. The charge in this case is not generated by a power pack. The tribogun is generally a long polytetrafluoroethylene tube. Friction is generated between the powder coating and the PTFE tube and a charge on the powder is generated by electron removal.

Other powder application techniques are known and could be used to apply the powder coating composition to the substrate. A technique that could be used is described in US Patent No. 6,342,273 (Handels, et al.). The technique involves first charging powder paint particles by friction or induction in the presence of carrier particles, feeding the charged paint and carrier particles to a transporter, transferring the charged paint particles from the transporter onto a transfer medium and applying the paint particles from the transfer medium to the substrate to form the powder coating.

It should be understood from the above that the powder coating process provides an environmentally friendly method of applying a coating as a solvent is not required and the overspray particles that are not bonded to a substrate can be collected and re-used in the next powder coating application.

A broad range of powder coating compositions and application methods could be used. A technique for powder coating the substrate of the invention involves the use of UV curable powder coatings. With such coatings, the powder is applied to the substrate and heated to and above the melting point of the powder coating composition. The temperature achieved in the melting phase is usually between 9O⁰C and 16O⁰C. The melting phase is conducted by either infrared (IR) heating oven or convection gas or electric heating oven, or a combination of the two systems. After melting and flow out of the powder stage the coated panel is then passed under a UV cure oven. At this stage the coating is irradiated with UV light. The UV light phase is generated by either a mercury lamp or a gallium doped mercury lamp with wavelengths of between 205 and 405 nm. Photo-initiators suitable for inclusion in UV powders include aromatic carbonyl compounds, such as benzophenone and alkylated or halogenated derivatives, anthraquinone and its derivatives, thioxanthone and its derivatives, benzoin ethers, aromatic or non-aromatic alphadiones, benzol dialkyl acetals, acetophenone derivatives and phosphine oxides.

The UV cure powder coatings can be applied to the substrate using similar techniques to standard coatings which require baking.

Combination UV and low temperature cure compositions use a combination of the two techniques described above. US Patent No. 5,922,473 and 6,005,017 include a more detailed description of such techniques.

Low bake powder coatings are designed to cure at temperatures between 9O⁰C (or less) and 16O⁰C for between 10 and 40 minutes total oven time (in a conventional gas or electric fired oven). IR cure of low bake powder will be much faster (from 30 seconds to 5 minutes). Epoxy and acrylic resins are commonly used.

All formulations should contain degassing agent. The degassing agent should be present in an amount of from 0.2 to 4% by weight. The Powdermate 542DG and Benzoin products are preferred as they generally provide good results, although others such as Oxymelt may be used. The Benzoin product is preferably present in an amount of from 0.3% to 1%, more preferably about 0.5%. The Powdermate product is preferably present in an amount of from 1% to 3%, more preferably about 1% by weight.

The formulations may optionally include a texture additive such as Bentone. The inclusion of a significant amount (2 to 15%, preferably 5 to 13%, most preferably 8 to 10% by weight) of texture additive is useful in formulations intended for use a primer coating. The coating can be sanded flat before the application of a top coat.

Some examples of some typical powder coating formulations are provided below. Tvpical UV Cure Powder Coating UV Cure Resin 50-95% UV Initiator 1-3% Pigments 1-30% depending on colour Additives 0-5% as required

The UV cured resin will generally have the following properties:

Glass transition Temperature of: 50 +/-1O⁰C Unsaturated equivalent weight 1300g/eq +/-200 Cone/Plate Viscosity (175⁰C) όOOOmPa.s +/-1000

The UV curing resin is generally an amorphous resin that can be cross-linked by a free radical polymerisation mechanism. The coating resin may contain unsaturated functional groups including methacrylic and acrylic unsaturated groups.

The UV curing resin may be polyester resin such as those commercially available as Uvecoat. A range of Uvecoat products are available including the 3002 product, which provide good weathering characteristics and some flexibility.

The UV initiators are added to start the free radical polymerisation mechanism, upon absorption of the UV irradiation energy. These can include but are not exclusive to α- hydroxyketone types such as l-(4-(2-Hydroxyethoxy)-phenyl)-2-hydroxy-2-methyl-l- propane-1-one and BAPO bis cyclophoshinoxide types such as Bis (2,4,6-trimethylbenzoyl)- phenylphosphineoxide). Example UV initiators include Irgacure 2959 and 819.

Other additives can be added as required.

Typical Low Temperature Cure Polyester Formulation Polyester 50-95% Cross-linker 1-16% Pigments 1-40% depending on colour Additives 0-5% as required

The polyester resin usually has the following properties:

Acid Value (or hydroxy value) 20-80 Viscosity 200-700 dPa.s (at 165⁰C) Glass Transition Temperature 50-70⁰C

A flow additive may be present in any of the compositions in an amount from 0 - 3%, typically 1%.

The present invention will now be described with reference to the following non-limiting examples.

Examples Unless otherwise indicated the mixtures were prepared by combining the ingredients (resins, initiators, colour pigments, extenders, flow additives and other minor additives). The mixture was then agitated and then heated and extruded at 100⁰C to provide a homogenous sheet. The sheet was cooled, granulated and then milled and sieved to provide particles having a particle size less than 125 micrometers (average particle size of 40 microns) to provide the powder coating composition. All amounts are parts by weight.

The powder coating compositions was applied electrostatically to the substrate material and cured. UV curing used Hg (Mercury) and Hg/Ga (Gallium Doped Mercury) UV lamps. Heat curing involved the use of an IR oven or a convection oven. Table 1. - Single Coat UV Cure Formulations

Coating formulation 1 was applied to plywood which had been preheated for 5 minutes at 7O⁰C, then cured by IR for 2 minutes at IR setting of 200⁰C and with 3 passes of UV.

The coated plywood panel was examined and found to provide a hard (no marring) and smooth surface without voids or dimple effects. The coated surface of the panel was water tested with steam for 3 minutes and examined. No water damage was noted. Coating formulation 2 was applied to plywood which had been preheated for 5 minutes at 7O⁰C, then cured by IR for 2 minutes at IR setting of 200⁰C and with 3 passes of UV. The coated plywood panel was examined and found to provide a hard (no marring) and smooth surface without voids or dimple effects. The coated surface of the panel was water tested with steam for 3 minutes and examined. No water damage was noted.

The formulations 3-5 were used as a primer coat for plywood. The plywood was preheated for 5 minutes at 7O⁰C, before the powder formulation was applied. The coatings were then cured by heating for 2 minutes at IR setting of 200⁰C and subjected to two passes of UV. The primer coatings were lightly sanded in preparation for a subsequent top coat.

All formulations were considered to be good UV Primer formulations. The appearance was good after sanding with no signs of pin holing from volatile or oil emissions from the plywood surface.

Coating formulation 6 was applied to and cured on plywood that had been primed and sanded with coating formula's 3-5 respectively. The primed plywood was preheated for 5 minutes at 7O⁰C, then by IR for 2 minutes at IR setting of 200⁰C and with 3 passes of UV.

All of the coated plywood panels were examined and found to provide a hard (no marring) and smooth surface without voids or dimple effects. The coated surface of each of the panels was water tested with steam for 3 minutes and examined. No water damage was noted.

Table 3. - Single Coat Low Temperature Cure Formulations

Coating formulations 7 and 8 were cured for 20 minutes at 15O⁰C. The coated substrates were examined and found to provide satisfactory results. The coated plywood panel was examined and found to provide a hard (no marring) and smooth surface without voids or dimple effects. The coated surface of the panel was water tested with steam for 3 minutes and examined. No water damage was noted.

Table 4 - Two Layer Low Temperature Coating Formulations

The coating formulations 9, 10 and 11 were applied to preheated plywood test panels (8 minutes at 9O⁰C) and heated for 20 minutes at 15O⁰C. The coating was then sanded to a smooth finish and top coated with coating formulation 12 and heated at 20 minutes at 15O⁰C. The coated substrates were examined and found to provide satisfactory results.

The coated plywood panel was examined and found to provide a hard (no marring) and smooth surface without voids or dimple effects. The coated surface of the panel was water tested with steam for 3 minutes and examined. No water damage was noted.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form or suggestion that that prior art forms part of the common general knowledge in Australia. It would be appreciated by a person skilled in the art that variations and/or modifications may be made to the invention as described without departing from the spirit or scope of the invention as broadly described. The described embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

1. Outdoor signage comprising powder coated MDF or plywood.

2. The signage of claim 1 comprising a plywood or MDF core and a weather protective and water resistant powder coating on the core on which information may be presented.

3. The signage of claim 1 or 2 wherein the powder coating composition used to form the powder coating on the MDF or plywood comprised a degassing agent in an amount of from 0.2 to 4% by weight of the powder coating composition.

4. The signage of claim 3 wherein the degassing agent is Powdermate 542DG and comprised from 1% to 3% by weight of the powder coating composition.

5. The signage of claim 4 wherein the degassing agent is Powdermate 542DG and comprised approximately 1% by weight of the powder coating composition.

6. The signage of claim 3 wherein the degassing agent is Benzoin product and comprised from 0.3% to 1% by weight of the powder coating composition.

7. The signage of claim 3 wherein the degassing agent is Benzoin product and comprised approximately 0.5 % by weight of the powder coating composition.

8. The signage of any one of claims 1 to 7 wherein the powder coating composition also contained a texture additive.

9. The signage of claim 8 wherein the texture additive comprised from 2 to 15% by weight of the powder coating composition.

10. The signage of claim 8 or 9 wherein the texture additive is Bentone.

11. The signage of claim 1 wherein the powder coating composition used to form the powder coat has the formulation set out in any one of examples 1 to 12.

12. A powder coating composition for use in forming a powder coat on MDF or plywood having the formulation set out in any one of examples 1 to 12.

13. Use of a powder coating composition as defined in claim 12 in the manufacture of outdoor signage by applying the composition to MDF or plywood and curing the composition to provide powder coated MDF or plywood.

14. The use of powder coated MDF or plywood according to claim 13 in outdoor signage.

15. A method of providing MDF or plywood signage suitable for outdoor use comprising applying a powder coating composition to MDF or plywood, curing the powder coating composition to provide powder coated MDF or plywood, optionally applying a further coating of a powder coating composition to the powder coated MDF or plywood and curing the further coating, wherein the method includes the step of modifying the powder coated MDF or plywood so that it present is capable of presenting information to a person who views the powder coated MDF or plywood.

16. The method of claim 15 wherein the powder coated MDF or plywood is shaped in manner which is capable of conveying information to a person who views the sign.

17. The method of claim 15 or 16 wherein the powder coated MDF or plywood has information presented on a surface of the powder coated MDF or plywood.

18. The method of any one of claims 15 to 17 wherein the powder composition used to form the powder coating on the MDF or plywood has the formulation set out in any one of examples 1 to 12.