Acoustic panel

Abstract

Acoustic panel 100 comprises a first layer 102 comprising non-woven hemp fibres, a second layer 104 comprising a hemp-derived material and an intumescent material 106 disposed in or on the acoustic pa

Description

BACKGROUND Acoustic panels or boards are used in construction to modify the acoustic properties of structures or rooms, especially to reduce sound transmission within or between spaces by absorbing energy from sound waves passing through the panels. Conventionally, the sound-absorbing materials which are used to form acoustic panels are principally synthetic materials such as compressed polyester, or materials from mineral sources such as mineral wool or fibre. The use of such materials is often considered environmentally harmful, for example due to emissions generated by production of the materials, the unsustainability of their sources, or a lack of bio-degradability. Hemp-derived materials have previously been used in the construction industry, for example as a more environmentally sustainable alternative to materials derived from non-renewable resources or using processes resulting in high levels of emission of carbon dioxide. Hemp-derived materials have good mechanical and thermal insulation properties, and the fast growth rate and high cellulose content of the plant make growth of the crop effective for sequestering carbon. The stalks of the hemp plant can be processed to separate the outer fibres from the inner core, which can then be broken into pieces known as shives. Hemp fibres have been used to create textiles, while hemp shives have been used as aggregate material. SUMMARY According to a first aspect, there is provided an acoustic panel or board comprising a first layer comprising non-woven hemp fibres, the first layer having a first density by volume, a second layer comprising a hemp-derived material, the second layer having a second density by volume that is different to the first density, and an intumescent material disposed in or on the acoustic panel. An acoustic panel which uses hemp-based materials may be more environmentally sustainable than previous alternatives. In particular, acoustic panels formed using hemp materials have been found to have a high degree of biodegradability. Additionally, as compared to panels formed of other biogenic materials (for example wood), panels formed using hemp-based materials may have greater potential for carbon sequestration over the lifetime of the product due to the carbon-dense nature of the crop. The acoustic panel additionally benefits from the advantageous mechanical and thermally-insulative properties of hemp materials, which allows the panel to be used in practical construction settings where it may be required, for example, to demonstrate a particular degree of resistance to fire. It has been found that hemp materials can be used to form a panel which is particularly effective for sound absorption by combining a non-woven hemp fibre layer with a second layer of hemp-derived material with a different density. The differing densities of the hemp materials in the two layers act to absorb sound waves across a spectrum of frequencies to a degree which is desirable for sound treatment. The panel comprises an intumescent material which increases in volume under exposure to heat. The inclusion of an intumescent material increases the resistance of the panel to fire (e.g. increasing the longevity of the material under exposure to flames), which can, for example, allow the panel to be used in certain applications where regulations dictate a minimum standard for resistance to fire. It has further been found that including an intumescent material can have an advantageous effect on the sound absorption properties of the panel. Adding an additional material with different properties to the first and second layers can lead to greater absorption of sound waves at different frequencies, and it has been found that using an intumescent material, in particular, combines favourably with the layer of non-woven hemp fibres and the second hemp-derived layer to absorb sound waves across a range of frequencies desirable for application in sound treatment. The intumescent material may be disposed within one of the first and second layers, for example as multiple disparate regions of intumescent material, or as an admixture of a binder within one or both of the first and second layers. Alternatively, the intumescent material may comprise an intumescent layer between the first and second layers. In embodiments, the intumescent material comprises a coating comprising an intumescent material which is disposed on an outer surface of the acoustic panel. Applying an intumescent layer as a coating has been found to lead to the above improvements in the properties of the panel without greatly increasing a thickness of the panel, which is particularly desirable in sound treatments. In particular, the intumescent material may comprise a coating disposed on the first layer. The intumescent material may be applied as a coating to the nonwoven hemp fibres of the first layer so as to form at least part of an outer surface of the panel. In such an arrangement, the intumescent layer can act to cover or seal the relatively porous non-woven fibre layer which would otherwise form an outer surface of the panel. This covering or sealing has been found to further contribute to improvements in the sound absorption capability of the panel. The second layer comprises a density by volume that is different to a density by volume of the first layer. In some embodiments, the second layer has a density that is greater than a density of the first layer. Combining a less-dense layer of non-woven hemp fibres with a more-dense hemp-derived layer may form an acoustic panel which has favourable sound absorption qualities while also being thin enough to be used as an effective sound treatment. The hemp-derived material of the second layer may also be non-woven hemp fibres, similar to the first layer, but having a different density by volume. Alternatively, the second layer may comprise hemp shives and a binder. The second layer may be formed by combining the hemp shives with the binder and curing, thermosetting and / or pressing the binder. A combination of hemp shives and binder form a relatively rigid layer as compared to non-woven hemp fibres. By using hemp fibres in a first layer and hemp shives in a second layer, the panel makes use of two separate parts of the hemp stalk, and can therefore be manufactured with reduced by-product waste. The binder may comprise a lime-based binder. A combination of hemp shives and a lime-based binder forms a material which is commonly referred to as hemp-lime or hempcrete. A layer formed from hemp shives and a lime-based binder may have particularly effective structural qualities such as a high rigidity. Alternatively, the binder may comprise a polymer. A combination of hemp shives and a polymer binder forms a wood-like material which may be referred to as hemp board, and which can be structurally similar to particleboard conventionally formed of wood. A combination of a first layer comprising non-woven hemp fibres with a second layer of hemp shives and a polymer binder has been found to form a particularly effective acoustic panel which demonstrates good sound absorption qualities, is relatively lightweight, and can be used to form a relatively thin panel. Such a panel may also be relatively easily worked by a user, for example in order to cut the panel down to size, without requiring extensive safety precautions. In other examples, the binder may comprise a combination of lime and polymer constituents. The polymer binder may comprise a resin. In particular, the polymer binder may comprise a biogenic resin, a resin having biogenic constituents, which may be referred to as a ‘bio-resin’. Using a resin derived from biological sources, combined with first and second layers of hemp-derived material, can provide an acoustic panel which has a desirably high degree of biodegradability in comparison to panels using greater amounts of synthetic resins or adhesives. The resin may be wholly or substantially (greater than 99% by weight) comprised of biogenic constituents. A binder comprising a biogenic resin may additionally comprise some synthetic, non-biogenic constituents. Preferably, the binder comprises biogenic constituents in an amount of at least 50% by weight of the total weight of the binder. More preferably, the binder comprises biogenic constituents in an amount of at least 70% by weight of the total weight of the binder. Most preferably, the binder comprises biogenic constituents in an amount of at least 90% by weight of the total weight of the binder. Where the second layer comprises a binder comprising a biogenic resin and comprises powdered carbon (as described below), the binder may comprise biogenic constituents in the any of the above amounts by weight, excluding the dry weight of the powdered carbon. Where the second layer is formed of a combination of hemp shives and a polymer binder, the binder may be present in amounts of 5% to 25% by weight of the total weight of the binder and hemp shives. More preferably, the binder may be present in amounts of 10% to 20% by weight of the total weight of the binder and hemp shives. The first layer and the second layer may be attached by a bond (rather than, for example, being mechanically attached). Where the intumescent material comprises an intumescent layer between the first and second layers, the first and second layers may each be attached to the intumescent layer by a bond. Where the panel comprises a third layer comprising non-woven hemp fibres between the first and second layers, as described below, any or all of the first, second and third layers and any intumescent layer may be connected to adjacent ones of the other layers by a bond as described herein. The bond may be formed by a polymer in the form of a resin or adhesive. In particular, the bond may be formed by or from a bio-resin as described above. In examples where the second layer comprises hemp shives and a binder, the bond may be formed by the same binder by composition as is present in the second layer, which may simplify the manufacturing process for forming the panel by allowing the second layer to be formed by the same mechanism which bonds layers of the panel together. The bond may be formed by a separate application of the same binder material by composition between the first and second layers. Alternatively, the bond may be formed by the binder which is present in the second layer; that is, the binder in the second layer may form a bond with the non-woven hemp fibres of the first layer and / or a binder in the first layer, if present. Alternatively, the bond may comprise a resin or adhesive according to any of the forms above, but different by composition to the binder of the second layer, which may still reduce the manufacturing complexity somewhat by allowing any bonding step to be carried out concurrently with the forming of the second layer from the binder, for example in a concurrent pressing or curing. The second layer may further comprise powdered carbon, for example powdered charcoal or activated carbon. The powdered carbon may comprise biogenic charcoal (‘biochar’) or biogenic activated carbon. Where a binder is used to form the second layer, the powdered carbon may be incorporated into the second layer as an admixture of the binder. The addition of powdered carbon can improve the sound absorption properties of the panel by altering the porosity of the layer. The addition of powdered carbon to a second layer formed from a combination of hemp shives and a binder has been found to be particularly effective in improving the sound absorption properties of the resulting panel, especially where the binder is a resin. The addition of biogenic powdered carbon can also increase the overall carbon sequestration of the panel, which can contribute to forming a more environmentally sustainable end-product. The first layer may further comprise a binder. The first layer may be formed of compressed non-woven hemp fibres and the binder. Compressing non-woven hemp fibres with a binder forms a material which may be referred to as a hemp mat. The first layer may be formed from a ‘pre-preg’ comprising non-woven hemp fibres imbued or covered with a binder. The non-woven hemp fibres may then be compressed, either with or without heating, to form the layer of compressed nonwoven hemp fibres and a binder. In some examples where the second layer is formed of a combination of hemp shives and a binder, the binder of the first layer may comprise the same material as the binder of the second layer. Alternatively, the binder of the first layer may comprise a different composition to the binder of the second layer. Alternatively, the first layer may be formed of compressed non-woven hemp fibres without a binder. The first layer may comprise a layer which is substantially wholly (greater than 99% by weight) formed of non-woven hemp fibres. It has been found that using a first layer formed of substantially only of non-woven hemp fibres can be particularly effective for sound absorption. Compressing the non-woven hemp fibres can allow the first layer to be formed with a desired degree of porosity and density, which can allow the sound absorption properties of the panel to be controlled. Compressing the non-woven hemp fibres can also allow the thickness of the panel to be controlled so that the panel may be used as an effective sound treatment. The panel may include a third layer comprising non-woven hemp fibres, the third layer arranged between the first layer and the second layer, and the third layer having a third density by volume that is different to both the first density and the second density. In particular, the third density may be greater than the first density and less than the second density. Adding a third layer of non-woven hemp fibres with a different density than the first and second layers can further improve the sound absorption properties of the panel across a range of different frequencies. The third layer may comprise compressed non-woven hemp fibres in the form described above in relation to the first layer. In particular, the third layer may comprise compressed non-woven hemp fibres without a binder. The third layer may comprise a layer which is substantially wholly (greater than 99% by weight) formed of non-woven hemp fibres. Alternatively, the third layer may comprise a binder in the same manner described above in relation to the first layer. In some examples, both of the first and third layers may comprise compressed non-woven hemp fibres and a binder. In embodiments, the binder may be the same by composition in both the first and third layers. The third layer may be compressed (with or without a binder) to a greater degree than the first layer to form a more dense layer. In embodiments, the first and third layers may comprise the same or substantially the same areal density (measured, for example, in grams per square meter) by being formed of the same or similar non-woven material, compressed to different degrees in order to comprise different volumetric densities. Particularly effective acoustic panels may be formed using a first layer comprising non-woven hemp fibres having an areal density of 600gsm to 1800gsm. More preferably, the first layer may comprise non-woven hemp fibres having an areal density of 800gsm to 1600gsm. In addition or alternatively, the third layer may comprise non-woven hemp fibres having an areal density of 600gsm to 1800gsm, or more preferably an areal density of 800gsm to 1600gsm. The acoustic panel may comprise a thickness of 20mm or greater. Panels formed according to any of the above and having thicknesses of at least 20mm may provide particularly effective sound absorption. Alternatively or additionally, the acoustic panel may comprise a thickness of 50mm or less. Panels having thicknesses of 50mm or less may have greater utility as sound treatments by providing effective sound absorption while remaining relatively thin for application over structural elements of a room. The acoustic panel may comprise a thickness of 20mm to 50mm. The acoustic panel may comprise a thickness of 20mm to 25mm or 45 to 50mm. Panels formed according to any of the above and having thicknesses in these ranges are effective sound treatments which are compatible with standardised systems for acoustic treatment. According to a further aspect, there is provided a system for acoustic treatment, the system comprising an acoustic panel according to any of the above, a track sized to receive an edge of the acoustic panel, and a covering layer configured to attach to the track and overlie the acoustic panel. In an assembled configuration, an edge of the acoustic panel is received in the track, and the covering layer is attached to the track and overlies the acoustic panel. The track may be attachable to a structural component of a building, for example to a wall of a room. The track holds the acoustic panel in place relative to the structural component in a location selected so that the acoustic panel may function as a sound treatment by absorbing sound waves. The covering layer overlies the acoustic panel to obscure the panel from view, which may allow the aesthetic qualities of system to be adjusted without requiring any material change to the acoustic panel which might compromise the sound absorption quality of the panel. In another aspect, there is a method of manufacturing an acoustic panel or board, the method comprising forming a first layer comprising non-woven hemp fibres, the first layer having a first density by volume, forming a second layer comprising a hemp-derived material, the second layer having a second density by volume that is different to the first density, and arranging an intumescent material in or on the acoustic panel. The method may comprise forming an acoustic panel or board according to any of the examples described above. The method may comprise forming a second layer in which the hemp-derived material is hemp shives, the method comprising forming the second layer from a combination of hemp shives and a binder. The step of forming the second layer may comprise arranging hemp shives together with the binder in a frame. The step of forming the second layer may comprise subsequently pressing the combination of the hemp shives and the binder within the frame. The pressing may comprise cold-pressing the combination of the hemp shives and the binder within the frame, i.e. performing the pressing in the absence of heating at or around ambient temperature. Alternatively or additionally, there may be performed a step of levelling the combination of hemp shives and the binder to form a more uniform upper surface of the composition. Alternatively to cold-pressing, or additionally and subsequent to coldpressing, the pressing may comprise hot-pressing the combination of the hemp shives and the binder within the frame. The hot-pressing may be carried out at temperatures of 100°C or greater, or more preferably at temperatures of 150°C to 200°C, with a particularly preferable 160°C to 180°C. The hot-pressing may be carried out in a static hot press. Alternatively, the hot-pressing may be carried out using a hot rolling press. Using a hot rolling press may be advantageous for forming acoustic panels, because the method may be particularly economical for forming relatively thin panels such as acoustic panels. The step of forming the first layer may comprise compressing the nonwoven hemp fibres to form the first layer. Alternatively, the step of forming the first layer may comprise arranging the non-woven hemp fibres together with a binder and compressing the non-woven hemp fibres and binder. The pressing may comprise either or both of a cold-pressing or a hot-pressing in any of the forms described above in relation to the first layer. The non-woven material may comprise a pre-preg, in which the non-woven hemp fibres are impregnated with the binder. Alternatively, the method may comprise adding the binder to the non-woven hemp fibres before compressing the non-woven material and binder. Alternatively, the compressing may be performed in the absence of a binder to form a first layer substantially comprising only nonwoven hemp fibres, as described above in relation to the form of the acoustic panel. The method may comprise carrying out any of the above steps of forming the first and second layers separately and subsequently bonding the first layer to the second layer. Alternatively, the method may comprise bonding the first layer and the second layer concurrently with any of the above steps of forming the first and second layers. In particular, the method may comprise arranging hemp shives together with a first binder in a frame, arranging the non-woven hemp fibres (without a binder) over the hemp shives and first binder, and hot pressing the hemp shives, first binder and non-woven hemp fibres to form first and second layers which are bonded together by the binder. The method may further comprise the step of forming a third layer comprising non-woven hemp fibres between the first layer and the second layer, the third layer having a third density by volume that is different to the first density and the second density. The step of forming the third layer of non-woven material comprising hemp fibres may comprise compressing the non-woven hemp fibres to form the third layer. Alternatively, the step of forming the third layer may comprise arranging the non-woven hemp fibres together with a binder and compressing the non-woven hemp fibres and binder to form the third layer. This step may be carried out separately to the steps of forming the first and second layers so that a degree of compression of the third layer may be controlled separately. The third layer may then be arranged between the first and second layers and bonded to both layers. Alternatively, the third layer may be bonded to the first and / or second layers concurrently with forming the first and / or second layers in any of the manners described above. The step of arranging the intumescent material in or on the acoustic panel may comprise applying a coating comprising an intumescent material to the first layer. The step may be carried out subsequent to both the steps of forming the first and second layers. The step may in one example comprise applying the coating comprising the intumescent material using a roller, a brush, or other contacting paint applicator. In one example, the step comprises spray-coating the first layer with the coating comprising the intumescent material. Spray-coating may allow a particularly effective degree of coverage of the intumescent coating over the first layer. The method may further comprise forming an acoustic panel comprising powdered carbon. In particular, the step of forming a second layer may comprise arranging hemp shives together with the binder in a frame, wherein the binder comprises the powdered carbon. BRIEF DESCRIPTION OF THE DRAWINGS There will now be described various non-limiting examples with reference to the drawings, in which: Fig. 1 shows an example acoustic panel having a first layer and a second layer; Fig. 2 shows another example acoustic panel having a first layer, a second layer and a third layer; and Fig. 3 shows a system for acoustic treatment comprising the acoustic panel of Fig. 1. DETAILED DESCRIPTION Fig. 1 shows an example acoustic panel 100. The acoustic panel 100 comprises a first layer 102 and a second layer 104. The first layer 102 comprises non-woven hemp fibres. In some examples, the first layer 102 further comprises a binder, and the non-woven hemp fibres are compressed along with the binder to form the first layer 102. The second layer 104 comprises a hemp derived material. In some examples, the hemp-derived material is non-woven hemp fibres. In other examples, the hemp-derived material is hemp shives. In some examples, the second layer 104 further comprises a binder. The acoustic panel further comprises an intumescent material 106. In the example shown, the intumescent material 106 comprises an intumescent coating applied over the first layer 102. In the example shown, the first layer 102 and the second layer 104 are attached by a bond 108. The bond 108 may be formed by the binder of the second layer 104, for example. In the example shown, the second layer 104 further comprises powdered carbon 110. The powdered carbon 110 is incorporated into the binder of the second layer 104. Fig. 2 shows another example acoustic panel 200. The panel 200 comprises a first layer 202, a second layer 204, and an intumescent material 208 comprising an intumescent coating applied over the first layer 202, similar to the panel 100 of Fig. 1. The panel 200 further comprises a third layer 206 disposed between the first layer 202 and the second layer 204. The third layer 206 comprises non-woven hemp fibres. In some examples, the third layer 206 further comprises a binder, and the non-woven hemp fibres are compressed along with the binder to form the third layer 206. The panel 200 comprises a first bond 210 between the first layer 202 and the third layer 206, and a second bond 212 between the second layer 204 and the third layer 206. With reference to Fig. 1 and continued reference to Fig. 2, the acoustic panels 100 and 200 each comprise a thickness t. The thickness t is the smallest dimension of the panel 100, 200, i.e. smaller than a width or a length of the panel 100, 200. In some examples, the thickness t is 20mm or greater. In some examples, the thickness t is 50mm or less. In some examples, the thickness t is 20mm to 25mm or 45 to 50mm. Fig. 3 shows an acoustic treatment system 300 incorporating the acoustic panel 100 of Fig. 1. The treatment system 300 comprises a track 302 receiving an edge of the panel 100. The track 302 comprises first and second legs 304, 306 extending from one another to form an ‘L-shaped’ cross section. The edge of the panel 100 is received within the square or rectangular region which is partially delimited by the first and second legs 304, 306. The panel 100 is received on the track 302 with one face of the panel 100 in contact with the second leg 306. The system 300 further comprises a covering layer 310. The covering layer 310 is attached to the track 302 and overlies the panel 100. The covering layer 310 is attached to the track 302 at an end 308 of the first leg 304. The system 300 further comprises a spacer 312. The spacer 312 is received between the first leg 304 of the track 302 and the edge of the panel 100. The spacer 312 may comprise a resilient, compressible member which bears against the first leg 304 and the edge of the panel 100. The track 302 may be attached to a structure at either the first leg 304 or the second leg 306 to secure the sound treatment system 300 in place relative to the structure. The system 300 may further comprise a second track (not shown) which receives an opposed edge of the panel 100. The second track may similarly be attached to the structure. Example One exemplary method of forming an acoustic panel will now be described. A frame was prepared having a length of 1200mm and a width of 600mm. A mixture of hemp shives and Cambond© resin was prepared in the frame with the resin forming approximately 10-15% of the mixture by weight. A layer of non-woven hemp fibres having a thickness of 7mm and an areal fibre density of 800gsm, without a binder, was laid over the mixture of hemp shives and resin. A static hot press was used to press the layer of non-woven hemp fibres with the mixture of hemp shives and resin for around 4-5 minutes at a temperature of around 160°C - 180°C to an overall panel thickness of 23mm. A coating of Envirograf® HW01 intumescent coating (product number 42), a water-based coating comprising melamine, was then applied to the non-woven hemp fibre surface of the panel by spray coating, and air-cured for 1 hour. The various embodiments described herein are presented only to assist in understanding and teaching the claimed features. These embodiments are provided as a representative sample of embodiments only, and are not exhaustive and / or exclusive. It is to be understood that advantages, embodiments, examples, functions, features, structures, and / or other aspects described herein are not to be considered limitations on the scope of the invention as defined by the claims or limitations on equivalents to the claims, and that other embodiments may be utilised and modifications may be made without departing from the scope of the claimed invention. Various embodiments of the invention may suitably comprise, consist of, or consist essentially of, appropriate combinations of the disclosed elements, components, features, parts, steps, means, etc, other than those specifically described herein. In addition, this disclosure may include other inventions not presently claimed, but which may be claimed in future.

Claims

1. An acoustic panel comprising:a first layer comprising non-woven hemp fibres, the first layer having a first density by volume;a second layer comprising a hemp-derived material, the second layer having a second density by volume that is different to the first density; andan intumescent material disposed in or on the acoustic panel.

2. The acoustic panel of claim 1, wherein the intumescent material comprises a coating comprising an intumescent material.

3. The acoustic panel of claim 2, wherein the coating is disposed on the first layer.

4. The acoustic panel of any of claims 1 to 3, wherein the intumescent material comprises a material selected from the group of: ammonium polyphosphate, melamine, sodium bicarbonate, or combinations thereof.

5. The acoustic panel of any preceding claim, wherein the hemp-derived material of the second layer is hemp shives, and the second layer further comprises a binder.

6. The acoustic panel of claim 5, wherein the binder comprises a polymer.

7. The acoustic panel of claim 6, wherein the binder comprises a resin.

8. The acoustic panel of claim 7, wherein the binder comprises biogenicconstituents in an amount of at least 50% by weight.

9. The acoustic panel of any preceding claim, wherein the first layer and the second layer are attached by a bond.

10. The acoustic panel of any of claims 5 to 8, wherein the first layer and the second layer are attached by a bond formed by the binder of the second layer.

11. The acoustic panel of any preceding claim, wherein the acoustic panel further comprises powdered carbon.

12. The acoustic panel of any of claims 5 to 8 or 10, wherein the acoustic panel further comprises powdered carbon within the binder of the second layer.

13. The acoustic panel of claim 11 or 12, wherein the powdered carbon is powdered biogenic charcoal or biogenic activated carbon.

14. The acoustic panel of any preceding claim, wherein the second density is greater than the first density.

15. The acoustic panel of any of claims 1 to 13, further comprising a third layer comprising non-woven hemp fibres, the third layer arranged between the first layer and the second layer, the third layer having a third density by volume that is different to the first density and the second density.

16. The acoustic panel of claim 15, wherein the second density is greater than the third density, and the third density is greater than the first density.

17. The acoustic panel of any preceding claim, wherein the panel comprises a thickness of 20mm or greater.

18. The acoustic panel of any preceding claim, wherein the panel comprises a thickness of 50mm or less.

19. A system for acoustic treatment, the system comprising:the acoustic panel of any preceding claim;a track sized to receive an edge of the acoustic panel; anda covering layer configured to attach to the track and overlie the acoustic panel.

20. A method of manufacturing an acoustic panel, the method comprising: forming a first layer comprising non-woven hemp fibres, the first layer having a first density by volume;forming a second layer comprising a hemp-derived material, the second layer having a second density by volume that is different to the first density; andarranging an intumescent material in or on the acoustic panel.

21. The method of claim 20, wherein the intumescent material comprises a coating comprising an intumescent material, the coating applied to the first layer by spray-coating.

22. The method of claim 20 or 21, wherein the step of forming the second layer comprises arranging the hemp shives in a frame with a binder and hot-pressing the hemp shives and the binder.

23. The method of any of claims 20 to 22, wherein the step of forming the first layer comprises compressing the non-woven hemp fibres and binder.

24. The method of any of claims 20 to 23, wherein the steps of forming the first layer and forming the second layer are performed concurrently.

25. The method of any of claims 20 to 24, further comprising the step of bonding the first and second layers concurrently with the steps of forming the first layer and forming the second layer.

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