EP1766175A1

EP1766175A1 - Structural metal element with improved thermal properties and a method for making same

Info

Publication number: EP1766175A1
Application number: EP05761428A
Authority: EP
Inventors: Merete Hallenstvet; Werner Jager; Eberhard Ackermann; Tore KOLÅS; Cornelis Spooren; Jostein MÅRDALEN
Original assignee: Norsk Hydro ASA
Current assignee: Hydro Extruded Solutions AS
Priority date: 2004-06-23
Filing date: 2005-06-23
Publication date: 2007-03-28
Also published as: US20080086973A1; BRPI0512518A; NO20070402L; CN1997805A; WO2006001708A1; AU2005257675A1

Abstract

Structural metal element for door-, window or the similar applications with improved thermal properties, having at least one interior surface, one exterior surface and one internal surface, where at least one surface of the element have improved optical properties by means of a surface treatment or coating. The surface in accordance with the invention has improved thermal emissivity and/or solar reflectance. Further, the invention also relates to a method of making the structural element, treated in a preferred manner or with preferred coatings.

Description

Structural metal element with improved thermal properties and a method for making same

The present invention relates to structural metal elements, in particular extruded elements to be applied in window-, door- or similar frames with improved thermal properties. The invention also relates to a method for making such an element, and may also be applied to facade elements such as wall- or roof- elements.

From DE 44 26 331 A1 it is known an extruded polymer profile to be used in door- or window- frames, with improved heat insulation properties. In one inner section of the profile there can be inserted insulating strips or metal inserts for increasing the stability of the profile. Further, it is suggested to coat selected surfaces of the profile with a reflective coating that reflects radiation in the infrared region of the spectrum. The coated surfaces facing outwards will reflect heat from the outside and keep the interior cooler in the warm season. Similarly, the coated surfaces facing inwards will reflect heat from the inside and keep the interior warmer in the cold season.

In accordance with the present invention there now can be made structural metal elements with improved thermal properties. In particular the improvements relates to structural elements of aluminium or an aluminium alloy. The structural element can be represented by an extruded profile or an assembly of such profiles for door- or window- frames, or other structural elements such as facade panels for wall- or roof- applications.

In the present document the following definitions will apply; a half shell is called a section. Out of two sections (for instance one inner and one outer section) and a thermal break bar one makes a thermally broken profile. A frame is a part that is fixed (e.g. screwed) into a building. A sash is a profile that is possible to open. Both the frame and the sash could be thermally broken or not. In the present embodiments, they are thermally broken. Further, the structural element may have one outside exposed to outdoor conditions named exterior surface, one inside called interior surface, and plural internal surfaces. In particular, in relation to sections carrying one window in it, the glazing bead area, the glazing rebate area and the thermal break surfaces have shown to be of importance regarding the overall thermal properties of the section in accordance with the present invention. The terms emittance or emissivity (ε) are referring to the ability of a surface to irradiate (emit) electromagnetic radiation. Solar reflectance (or reflectivity) refers to the ability of a surface to reflect solar radiation.

The reflectance (p) and emittance (ε) of a surface can vary strongly with wavelength (λ) of radiation. The emittance (ε) of an object is defined as the ratio of radiant energy emitted by the object to that of a perfect Planckian blackbody radiator at the same temperature and wavelength, that is, an object following Planck's law. Thermal emittance is determined by a weighing process of the emittance, ε(λ), in the thermal wavelength region of the electromagnetic spectrum, see Figure 1.

For opaque materials with no transmittance, the relation between emittance and reflectance simplifies to the following: ε(λ) = 1 -p(λ)

Compared to polymers, metals have a significantly higher thermal conductivity. As a result of this, metal window- and doorframes in particular have been associated in the past with significant conductive heat losses.

In modern metal window- and doorframes, polymer strips with low thermal conductivity are being integrated into the frames in order to thermally separate an inner and outer frame section (thermal breaks). This significantly reduces the thermal loss by conductivity. In aluminium frames with proper application of thermal breaks, up to 65% of the remaining heat transfer through the frame is caused by thermal radiation between the inner and outer sections. A further improvement of the thermal characteristics of the aluminium frame thus requires the minimization of radiative heat transfer losses. The heat transfer caused by thermal radiation is governed by the thermal emittance properties of the section surfaces. To reduce thermal heat transfer it is especially important that the thermal emittance of the internal and interior surfaces of the structural element is kept as low as possible. The emittance of the exterior surface is of less importance due to larger heat transfer by convection at the exterior surface.

Solar radiation contains a significant amount of energy; see Figure 2. When direct sun radiation is absorbed in a structural element the temperature increases. In aluminium frames, this solar heat gain can be a problem for several reasons.

First of all, solar heat gain will increase the amount of excessive heat that is transported into the building interiors. Such absorption of solar energy and heat transfer through building materials may have a negative impact on the cooling load and personal comfort during the warm season.

Secondly, the heat gain can result in thermal expansion of the metal. The thermal expansion properties of aluminium are strongly influencing the behaviour of large window constructions. In direct sunshine the aluminium window frames will expand due to solar heating. This expansion can result in twisting of the window section and the window can be difficult to open. The solar heat gain properties of a structural element are influenced by the solar reflectance of the surfaces that are exposed to solar radiation, as well as the thermal emittance properties of the other surfaces. To reduce solar heat gain, the solar reflectance of the exterior surface should be as high as possible. The thermal emittance of the exterior surface is of less importance. The reason for this is that the heat loss from the exterior surface is dominated by heat convection and conduction, rather than emittance.

Prior art solar reflective coatings The solar reflectance of a surface is the fraction of the incident solar energy, which is reflected by the surface in question. The best standard technique for its determination uses spectrophotometric measurements with an integrating sphere to determine the reflectance at each wavelength. The total solar reflectance (TSR) is determined by a weighing process, using a standard terrestrial solar spectrum (see figure 2). ASTM E903 and E892 document this method.

The solar spectrum consists of three wavelength-regions and the energy is distributed approximately as follows:

Ultra violet region < 400 nm 5% Visible region 400 - 700 nm 44% Near infrared region 700 - 2500 nm 51%

Therefore, to reduce solar heat gain, the reflective surface properties in near infrared region, as well as the visible and UV regions are of importance. The reflectance in the visible region will determine the visual appearance (colour and brightness) of the surface. Traditionally, solar heat gain is most problematic for black (or other dark coloured) surfaces. A black surface typically has a low reflectance in all parts of the solar spectrum. However, it is possible to produce a black surface with a much higher solar reflectance by altering the reflectance properties in the UV and near infrared parts of the solar spectrum. Since these parts of the spectrum are not visible to the human eye, such a modification of the surface reflectance properties will not alter the visual appearance of the surface.

Traditionally only white and light shade coloured coatings have given a relatively high solar reflectance. Special near infrared reflective coatings are a recent development that has found a main application as outside coatings on metal roofs. These paints keep the roof relatively cool, and allow at the same time a wide choice of roof colours. With near- infrared-reflective pigments, even the solar reflectance of black coatings has been increased to more than 25%, whereas conventional black coatings typically give a solar reflectance of only 5%. Powder coatings with these pigments have so far not been commercially available.

Prior art low emissive coatings Low emissive coatings are known to have been applied to the surface of building elements for improved thermal performance. One example is the application of low emissive coating on the inside of metal roofs. These types of low emissive coatings were initially developed for military applications. The low emissive surface of, for example a military vehicle, can alter and suppress the thermal radiation from the object and make it harder to detect with infrared sensors. Such low emissive coatings are typically produced with the use of metallic pigments.

Aluminium has a high reflectance in the thermal region of typically 0.9; corresponding to a thermal emissivity of 0.1 , see Figure 1. For this reason, metallic aluminium flakes are commonly used as pigments in such low-e coatings.

Element with improved thermal properties (invention) Aluminium building products are usually surface treated in order to yield an appropriate durability and appearance, without particularly addressing the emissive or solar reflective properties. Further, the surface treatment needs to comply with the standards in the market field (eg. GSB International, Qualicoat or Qualanod). For most surface treatments it is very difficult to combine outdoor durability, scratch resistance, acceptable visual appearance, acceptable production cost and other standard requirements with low emissivity. Normally the metal is anodised or powder coated. This gives excellent outdoor durability but high emissivity.

Standard surface treatment procedures applied today typically result in thermal emissivities of ε=0, 85-0,9 on all surfaces. According to the invention it is possible to improve the thermal performance of the said structural metal element by applying coatings with optimised optical properties on the various surfaces, while at the same time fulfilling the GSB and/or Qualicoat and/or Qualanod requirements.

The improvements of the present invention are based upon the fact that the thermal emittance as well as the solar reflectance of the various surfaces will influence the thermal properties of a structural metal element. Further, the desired solar reflective and thermal emissive properties of the exterior surface can be different from that of the internal and interior surfaces.

In accordance with the invention, a structural metal element with improved thermal properties can be made by improving the optical properties (emissivity and solar reflectance) of the various surfaces. The internal and interior surfaces should preferably have a low thermal emittance. The exterior surfaces should preferably have a high solar reflectance. The thermal emittance of the exterior surface is of less importance due to larger heat transfer by convection at the exterior surface.

Further, the invention describes three different procedures to achieve such improved optical properties for an element consisting of two (one inner and one outer) or more separated metal sections. The first procedure (A) is to treat all surfaces with the same (improved) treatment, resulting in low emissivity and/or high solar reflectance on all surfaces. The second procedure (B) is to treat the inner and outer sections with different (improved/optimised) surface treatments, giving low emissivity for the inner section(s), and/or high solar reflectance on the outer section(s). The third procedure (C) is to treat the various surfaces on each section differently, resulting in, for example low emissivity on the internal and interior surfaces and/or high solar reflectance on the exterior surfaces.

The present invention also takes advantage of the fact that metal surfaces in general, and aluminium surfaces in particular have especially favourable solar reflective and low emissive properties (see Figure 1 ) that can be utilised to improve the thermal properties of a structural element that is made of such metal.

Still further, in accordance with the invention it is proposed several surface treatment schemes enhancing the thermal properties of structural metal elements. The invention is based upon the fact that the heat transport caused by radiation through the sections is reduced by means of improving the surface properties of the sections. The surface properties may be altered by means of a low emissive and/or solar reflective coating scheme. This could also include utilizing the inherent low emissive and solar reflective properties of the metal substrate as such.

The above mentioned and further advantages can be achieved in accordance with the invention as defined in the accompanying claims.

The invention shall be further explained by examples and figures where:

Rg. 1 discloses the reflectivity of aluminium in the UV, Visible and IR region of the electromagnetic spectrum,

Fig. 2 discloses the terrestrial solar spectral energy density measured (at the earth surface) according to ASTM E489,

Hg. 3 discloses the cross section of a window section with the different surfaces,

Fig. 4 discloses the reflectance spectra of low emissive powder coating and low emissive paint,

Fig. 5 discloses the reflectance spectra of enhanced barrier type aluminium oxide,

Fig. 6 discloses the reflectance spectra of a sol gel coating,

Fig. 7 discloses the reflectance spectra of low emissive foil,

Fig. 8 discloses the reflectance spectra of black solar heat reflecting black coating.

Examples of improved thermal properties According to the invention, several proposed surface treatment schemes are shown to significantly improve the emissivity of the structural metal element surfaces. Further, it is shown that the resulting U-values of the metal frame as a whole can be significantly influenced by the various treatment processes.

The preferred solution to improve the thermal properties of the structural element is to use a surface treatment scheme that improves the surface properties of the structural element. The internal and interior surfaces should preferably have a low emissivity. The exterior surfaces should preferably have a high solar reflectance. Fig. 3 discloses the cross section of a window section with the different surfaces (a window frame 1 with concealed sash). A window pane 10 is supported by glazing gaskets 3 and 4. The gasket 3 is supported by a glazing bead 2. Reference numeral 9 denotes a central gasket, while reference numeral 6 denotes a stop gasket. One or more thermal breaks 11 may be arranged in the frame. Sections 5, 7 and 8 are preferably made out of aluminium. The window frame further has at least one interior surface SI and one exterior surface SE. Further, there is shown internal surfaces such as one glass rebate area F, glazing bead area SG, and thermal break surfaces STB. Experiments carried out with different surface treatment processes at said surfaces have shown that the U-value of the frame as a whole can be highly influenced by various treatment processes. This is due to the optimised emissivity of the various surfaces of the frame.

Table 1 shows the optical properties of various coatings.

Table 1

The following examples refers to table 2: Example 1 and 2 describes the prior art of surface treatment of aluminium profiles for building applications.

In Example No 1 standard anodising of 20 - 25//m is used, resulting in a surface emissivity of 0.85 on all surfaces. In example No 2 a standard powder coating in white or any other solid colour is applied resulting in an emissivity of 0,9 on all the powder coated surfaces. The surfaces in the thermal brake area are not coated and retain the emissivity of the aluminium substrate. For an aluminium window frame with a concealed sash as shown in figure 3, the resulting U-value of the frame with prior art surface treatments will be 4.2 - 4,5 WVm²K (see table 3). Abbreviations: SE = Exterior surface, SI = Interior surface, SG = Surface in the glazing area, STB Surface of the thermal brake area Table 2 Example No. 3 consists of anodising the inner section with a thin anodising layer of 2 μm thickness. Such a thin layer will be semitransparent for thermal radiation. Therefore, the superior low emissive properties of the underlying metal substrate will not be completely suppressed. On standard extruded aluminium it is shown that such a coating gives an emissivity of 0.4. The outer section is surface treated in standard anodising. The sections are anodised before the thermal brake bar is inserted.

Example No 4 consists of barrier type anodising layer on the inner section with a layer thickness of 0,4 - 0,7//m and a standard anodising 20 - 25μm on the outer section. The barrier type coating has an emissivity 0,15 - 0,30 depending on the gloss of the initial surface. The sections are anodised before the thermal brake bar is inserted

Example No. 5: Standard powder coating of a thermal broken profile typically results in a surface emissivity of 0.9 for the coated surfaces, and 0.1 for the internal surfaces. In this example the standard powder coated profile is additionally painted in the glazing rebate area with a specially prepared low emissive wet paint with a typical emissivity of 0,2-0,3. For the frame in figure 3, the resulting U-vale of the frame is 3,95W/m²K (see table 3).

Example No 6: Standard powder coating of thermal broken profiles will not coat the internal surface resulting in an emissivity of 0,1 on these surfaces. The aluminium substrate's emissivity of 0,1 can also be achieved by removal of the powder coating from the glazing rebate area. Masking the critical areas before powder coating is one procedure to achieve powder coating free areas.

Example No 7: Profiles with standard powder coating can also be improved by applying a low emissive foil to the glazing rebate area. These foils will typical have an emissivity of 0,10. The foil can be glued to the powder coated surfaces. For the frame in figure 3, the resulting U value is 3,23 WYm²K (see table 3).

Example No. 8 consists of a specially prepared low emissive powder coating with emissivity 0.4 applied to the interior surfaces of the inner section and a standard powder coating with any colour on the exterior side of the outer section. All internal surfaces and critical glazing areas are protected during the powder coating operation and will retain the substrate's superior emissivity of 0,1. Example No 9 consists of a specially prepared low emissive powder coating with excellent outdoor performance and with an emissivity of 0,4- 0,5 applied on thermal broken profiles.

Example No 10 consists of a further application of a low emissive wet paint with emissivity of 0,2-0,3 in the glazing area.

Example No 11 consists of low emissive powder coating applied on the interior surface of the inner section and the exterior side of the outer section. The glazing areas are protected by masking during the coating application and will retain the substrate emissivity of 0,1.

Example No 12 consists of applying a low emissive foil with an emissivity of 0,1 in the glazing rebate area on profiles coated with low emissive powder coating.

Example No 13 consists of sol-gel coating applied to the inner section resulting in an emissivity of 0,15 on all surfaces of this section. Standard powder coating with any colour is applied on the exterior side of the outer section; the thermal brake surface and critical glazing areas are protected during the powder coating operation and will retain the substrate's superior emissivity of 0,1.

Example No 14 consists of sol gel coating on the entire thermal broken profile resulting in an emissivity of 0,15 on all surfaces.

Example No 15 consists of a solar heat reflecting powder coating on the exterior surface of outer section and a low emissive powder coating on the interior surface of the inner section. It should be understood that other low emissive surface treatment system like low emissive wet paint, thin film anodising, barrier type anodising layer or a sol-gel coating can be applied on the inner section.

Comparison of U-values for different coatings Table 3 below gives the surface emissivities influence on U-values of Wicline 50E Concealed sash with and without glass. The window glass of 1 ,48 x 1 ,23 metres have an U-value = 1 ,1 VWm²K. The typical emissivities of the various coating have been applied in the calculations: Emissivity: Metallic powder coating: 0,5 Wet aluminium pigmented paint: 0,3 New anodising process: 0,15 on glossy surface or 0,3 on matt surfaces Sol gel coatings: 0,15

Table 3

Experiments carried out with different surface treatment processes at said surfaces have shown that the U-value of the frame as a whole can be highly influenced by various treatment processes. This is due to the optimised emissivity of the various surfaces of the frame.

Low emissive powder coating and wet paint A low emissive powder coating and wet paint are necessary parts of several of the proposed surface treatment schemes. The preferred solutions are aluminium-pigmented coatings. Aluminium pigmented powder coatings will typically have a higher emittance than that of a wet paint due to restrictions of pigment volume concentrations in the application of powder coatings. If the pigment concentration in a powder coating exceed typical 5%, the powder coating will be difficult to electro static charge and spray on to the surface. The emittance of a typical low emissive powder coating can be 0.50 or less. The pigments would ideally be bonded to a resin matrix that absorbs little in the thermal region of the spectrum to enable low emissivity even with a low amount of pigments.

Low emissive wet paints have a higher pigment volume concentration than powder coatings and due to evaporation of solvents during drying and curing of the paint the resulting low e film from wet paint will contain much more pigments then films from low e powder coatings. High concentration of aluminium pigments in a film gives low emittance. The emittance of a typical powder-coating can be 0,20. Further, the invention is based upon the fact that coating dependent parameters such as amount, pigment size, pigment shape (leafing, round, flake), the localization of the coating layer and its thickness together with pigments and powder properties will influence the emissivity properties of a section.

Barrier aluminium oxide anodising In standard architectural anodising the electrolyte is normally sulphuric acid, which will partly dissolve some of the formed aluminium oxide leaving a porous aluminium oxide structures. This porous structure allows the anodising process to continue, as the oxide grows thicker; layer thickness of 25 μm can be formed at low voltage.

In barrier type anodising the electrolyte is changed to an electrolyte, which do not dissolve the electrolytic formed oxide. The voltage will increase as the resistance in the oxide increase. High voltage will be necessary to force the current trough the anodising layer. The electrolyte can be ammonium tartar, boric acid, ammonium pentaborate, or an organic acid. The barrier anodising film will have a film thickness of 400 - 700nm and the emissivity of the surface will be 0,15 - 0,30 depending on the roughness of the initial surface. The chemical resistance of a barrier type layer is much better than standard anodising due to the absent of the porous structure and its content of aluminium hydroxide and residuals of the electrolyte.

Low IR-absorbing coating on aluminium substrate The coating could be prepared by sol-gel technology or similar. As IR reflectors to assure low emissivity there could be used e.g metal particles in the nano scale or even the aluminium substrate as such, in combination with an extremely thin and/or IR-transparent coating layer. In one embodiment, the coating should be thick enough to protect the surface, but as thin and IR-transparent as possible so that it would not suppress too much the positive low emissive (and solar reflective) properties of the underlaying metal surfaces.

Low IR absorbing coating can be prepared by filling an organic or inorganic coating with nano sized IR transparent particles and example can be aluminium nitride

It is also conceivable with a spectrally selective coating scheme that takes advantage of the superior reflective properties of the underlying metal. If the nano coating is sufficiently transparent to radiation throughout the UV, visible, near IR (i.e the solar region) and thermal regions of the electromagnetic spectrum it could be possible to tailor the reflective properties of the surface by introducing nano scale particles/components into the coating that absorb only in selected parts of the spectrum. In this way both the thermal emissivity, the solar reflectance as well as the visual appearance of the surface can be modified and controlled.

Low emissive foils Low emissive foils are prepared by deposition of a reflective metal or metal oxide on a foil of polymer material or another non-reflective substrate. The reflective film is normally applied by chemical vapour deposition or physical vapour deposition. The reflective film can be in silver or other reflecting materials. The low emissive foil can be glued on to any other non reflecting surface.

Solar reflective powder coating Solar heat reflecting powder coating can be supplied in black, dark shades of grey, brown, green, blue and red. The measured total solar heat reflection (TSR) has a linear relation to the measured surface temperature. Black and dark RAL colours like RAL 6005, 8017 or 8011 are available with TSR > 20.

The pigments should preferably have a high reflectance in the near infrared region of the spectrum and also possibly in the UV-region, as this will not change the colour. An alternative strategy is to take advantage of the superior solar reflective properties of the underlying metal substrate. In this case, the coating should be transparent in the near infrared parts of the spectrum.

In the example above, there is shown a window frame with concealed sash. However, it should be understood that the principles of the invention may be applied for other structural elements as well, in particular applications involving a hollow/profile section with one interior and one exterior surface.

Claims

1. Structural metal element for door-, window or the similar applications with improved thermal properties, having at least one interior surface, one exterior surface and one or more internal surfaces, where at least one surface of the element have improved optical properties by means of a surface treatment or coating characterised in that the surface have improved thermal emissivity and/or solar reflectance.

2. Structural element in accordance with claim 1 , where said element is made out of at least one inner section and one outer section preferably with thermal breaks between internal surfaces of said sections, characterised in that the inner section can have different optical properties than that of the outer section.

3. Structural element in accordance with claim 1-2, characterised in that at least two surfaces of the same section can have different thermal emissivity.

4. Structural element in accordance with claim 1 -2, characterised in that at least one internal surface of the element have low emissivity properties.

5. Structural element in accordance with claim 4, characterised in that at least one internal surface of the structural element is coated with a low emissive, wet metallic paint.

6. Structural element in accordance with claim 4, characterised in that at least one internal surface is coated with a low emissive foil.

7. Structural element in accordance with claim 1-2, characterised in that the interior surface of the structural element have low emissivity properties, the emissivity is ε < 0,85.

8. Structural element in accordance with claim 1 -2, characterised in that the black or dark coloured exterior surface of the structural element have high solar reflectance properties TSR > 20.

9. Structural element in accordance with claim 1-2, characterised in that the emissivity in the thermal break area is ε < 0,85.

10. Structural element in accordance with claim 1-2, characterised in that the emissivity in the glazing bead area is ε < 0,85.

11. Structural element in accordance with claim 1 -2, characterised in that the exterior and/or the interior of the structural element is coated by a low emissive powder coating.

12. Structural element in accordance with claim 11 , characterised in that the coating is based upon a powder coating consisting of particles based upon a polymeric matrix having metal flakes adhered thereto.

13. Structural element in accordance with claim 1 -2, characterised in that the element in addition has enhanced reflectance properties in the NIR and visual region of the spectre.

14. Structural element in accordance with claim 1-2, having at least one surface in a glass bead area, characterised in that said surface have low emissivity properties.

15. Structural element in accordance with claim 1 -2, characterised in that it is made out of aluminium or an aluminium alloy.

16. Structural elements in accordance with claim 1-2, arranged to form one window- or door frame, characterised in that the resulting U-value of the window- or door frame is reduced by more than 6%.

17. A method of producing a structural element in accordance with claim 1-15 characterised in that the coating is applied in a powder coating process.

18. A method of producing a structural element in accordance with claim 1 -15, characterised in that the coating is applied in a co-extruding process.

19. A method of producing a structural element in accordance with claim 1-15, characterised in that the surface treatment consists ofa sol-gel coating.

20. A method of producing a structural element in accordance with claim 2-15, characterised in that the outer section is surface treated and/or coated to establish high solar reflectance properties.

21. A method of producing a structural element in accordance with claim 20, characterised in that the internal side of the outer section is masked in the treatment process.

22. A method of producing a structural element in accordance with claim 20, characterised in that the internal side of the outer section is coated with a low emissive paint or a low emissive foil.

23. A method of producing a structural element in accordance with claim 2-15, characterised in that the inner section is surface treated and/or coated to establish low emissivity properties.

24. A method of producing a structural element in accordance with claim 23, characterised in that the interior of the inner section is further surface treated and/or coated while the internal side of said section is masked.

25. A method of producing a structural element in accordance with claim 1 -15, characterised in that the surface is treated by a barrier aluminium oxide anodising process.