WO2012059713A1

WO2012059713A1 - Acoustic composite panels and methods for their manufacture

Info

Publication number: WO2012059713A1
Application number: PCT/GB2011/001546
Authority: WO
Inventors: Timothy John Sweatman; Douglas Montague Spinney
Original assignee: Ecotechnilin Limited
Priority date: 2010-11-03
Filing date: 2011-11-01
Publication date: 2012-05-10
Also published as: GB201018494D0; GB2485165A

Abstract

An acoustic panel has a sandwich construction with a core formed of acoustically absorbent material such as paper honeycomb. First and second skin layers are formed adhering to the core. The first and second skin layers comprise natural fibre and cured furan resin. The first and second skin layers have substantially different acoustic properties, in view of differences in their density. A method is disclosed for manufacturing the acoustic panel. In a curing step, to form the first and second skin layers, the skin layers are sequentially cured at different pressures in order to provide the different densities.

Description

ACOUSTIC COMPOSITE PANELS AND METHODS FOR THEIR MANUFACTURE

BACKGROUND TO THE INVENTION Field of the invention

The present invention relates to composite panels of interest for their acoustic properties, mechanical stiffness and light weight for use in construction. The present invention also relates to methods for the manufacture of such composite panels. The present invention has particular, but not exclusive, application to ceiling panels and tiles and wall panels for buildings.

Related art WO 2010/146355 discloses the manufacture of composite panels. The panels are formed with a core of a suitable material such as cork, paper or card. Resin-impregnated natural fibre mats are applied to the upper and lower faces of the core in a sandwich-type construction. The structure is pressed and heated to cure the resin. In this process, the resin-impregnated mat is compressed to form a skin layer. In WO 2010/146355, the resin is a sugar-based bio-resin, specifically furan resin. This resin has advantageous properties in terms of environmental impact, safety of handling, strength and fire retardance.

Cageao et al (2004) [RA Cageao, J Lorenzo and Franken "Studies of Composites Made with Baypreg® F: Component Selection for Optimal Mechanical Properties"

Polyurethanes October 18-20 2004, pp. 513-519] discloses the manufacture of a load bearing panel having a low density core with upper and lower skin layers formed of fibre reinforced polyurethane. The low density core may be formed from paper honeycomb. The skins are formed by impregnating a fibre mat such as a glass fibre mat with polyurethane resin and curing the resin. The dry fibre mat is first applied to the core and then the fibre mat is sprayed with polyurethane resin. The wet sandwich is then pressed and heated to cure the resin. The use of polyurethane resin presents very significant health and safety issues, since the components of the resin are known to be harmful to health - for example, isocyanates are known skin and respiratory sensitizers. Press curing of polyurethane resin typically requires the use of a sealed press. The finished product is disclosed being for use, for example, as a car boot load floor. SUMMARY OF THE INVENTION

It is widely acknowledged in the construction industry that further progress is necessary towards the use of materials and manufacturing processes that have a lower

environmental impact than those conventionally used. However, of course, building materials must conform to the relevant local buildings regulations and so it is required to provide new construction materials that have properties comparable to conventional constructional materials. For this reason, the present inventors have aimed to develop replacement panels for the construction industry that have a lower environmental impact, but corresponding or better technical performance, than known construction materials.

The present invention has been devised in order to address at least one of the above problems. Preferably, the present invention reduces, ameliorates, avoids or overcomes at least one of the above problems.

The present inventors have realised that it is possible to further develop the technology first disclosed in WO 2010/146355 in order to provide a composite panel formed using resin-impregnated natural fibre mats, the panel having excellent acoustic properties. A major realisation by the inventors is that it is necessary to consider separately the function of the two skin layers. In this way, the panel can be engineered so that the skin layers provide the panel with asymmetric acoustic properties.

Accordingly, in a first preferred aspect, the present invention provides a method for manufacturing an acoustic panel having a sandwich construction with a paper or card cellular core and first and second skin layers comprising natural fibre and cured bio-resin, wherein the method includes the steps:

(i) providing first and second natural fibre mats;

(ii) applying a first bio-resin to the first and second natural fibre mats to at least partly impregnate them with the first bio-resin;

(iii) providing a core formed of acoustically absorbent material;

(iv) applying a second bio-resin to the first and second natural fibre mats and/or to the core;

(v) applying the first and second impregnated natural fibre mat to opposing sides of the core, thereby forming a wet sandwich construction; and (vi) pressing and heating the wet sandwich construction so as to compress the impregnated natural fibre mats and cure the first and second bio-resins to form the first and second skin layers,

wherein the first and second skin layers have substantially different acoustic properties as a result of one or any combination of: different composition of bio-resin in the first and second mats; different viscosity of applied bio-resin in the first and second mats; different water content of applied bio-resin in the first and second mats; different quantity of bio- resin in the first and second mats; different curing temperature, different curing speed, and different applied pressure during curing.

In this way, the present invention allows the manufacture of a panel with a skin layer of relatively low acoustic reflectance and a skin layer with a relatively high acoustic reflectance. A low acoustic reflectance can be considered to correspond to a high acoustic porosity and a high acoustic reflectance can be considered to correspond to a low acoustic porosity. Positioning the panel with the low reflectance skin facing towards a room allows sound to enter the panel but to be reflected from the high reflectance skin. The reflected sound is dissipated in the core of the panel, leading to excellent acoustic performance. The core may be formed of a paper or card cellular structure, e.g. a honeycomb structure. Such a core is found to have suitable acoustic absorbing properties. Alternatively, the core may be formed of a foam core, particularly a foam core selected for its fire-resistant properties. In a second preferred aspect, the present invention provides an acoustic panel obtained by or obtainable by the method of the first aspect.

In a third preferred aspect, the present invention provides an acoustic panel having a sandwich construction with a core formed of acoustically absorbent material and first and second skin layers comprising natural fibre and cured bio-resin, wherein the first and second skin layers have substantially different acoustic properties.

In a fourth preferred aspect, the present invention provides a laminated board wherein an acoustically absorbent core material has surfaces of resin-impregnated fibre material one being significantly less porous to sound than the other. One of the resin-impregnated surface layers may be cured in a multiplicity of stages to give an acoustically porous or semi-porous surface. The acoustic, thermal, and physical stiffness properties may be adjusted according to need by varying the thicknesses of the layers, the pressure applied and the curing times given to each stage of manufacture. The first, second, third and/or fourth aspects of the invention may be combined with one or more of each other. Furthermore, they may have any one or, to the extent that they are compatible, any combination of the following optional features.

The present inventors have found in particular that skins of different acoustic properties may be formed depending on the pressure applied to the skins during the curing step. It is speculated, without wishing to be bound by theory, that curing the resin-impregnated mats under relatively low pressure can form a relatively porous, relatively low density structure that has low acoustic reflectance (i.e. high porosity to sound). On the other hand, curing the resin-impregnated mats under relatively high pressure compresses the mat further and leads to the formation of a relatively non-porous, relatively high density structure that has high acoustic reflectance (i.e. low porosity to sound).

Accordingly, it is a preferred feature of the invention that the first skin layer has a lower density than the second skin layer.

Furthermore, it is a preferred feature of the method of the invention that the pressure applied during curing of the first resin-impregnated mat is different to the pressure applied during curing of the second resin-impregnated mat. For example, curing may be carried out by sequential heating so that the first resin-impregnated mat is subjected to a curing temperature before the second resin-impregnated mat is subjected to a curing temperature. By "curing temperature" here it is intended to refer to a temperature which is sufficiently high to commence curing of the first bio-resin. Thus, in a first curing stage, the first resin-impregnated mat is subjected to a higher temperature than the second resin-impregnated mat so that the first resin in the first resin-impregnated mat begins to cure whilst the first resin in the second resin-impregnated mat does not. Typically, the two resin-impregnated mats are subjected to the same pressure in the first curing stage, because the most straightforward way to apply pressure is by pressing the sandwich between opposing faces of a mould. In a second curing stage, the second resin- impregnated mat is subjected to a higher temperature than in the first curing stage in order to cure the first resin in the second resin-impregnated mat. The pressure applied in the second curing stage is higher than in the first curing stage, and as in the first curing stage the pressure applied to each mat is the same due to the typical method of pressing. The temperature applied to the first mat during the second moulding stage may be similar to, lower than or higher than the temperature applied to the second mat during the second curing stage, depending on whether further curing of the first mat is required.

Curing can be carried out in a sequential moulding apparatus in which the first curing stage takes place between pressing members heated to different temperatures and in which the second moulding stage takes place between different pressing members, optionally heated to the same temperature. The sandwich is typically subjected to a greater pressure during the second curing stage than during the first curing stage. A particularly preferred apparatus for this is a double belt press, which allows for a substantially continuous process.

Alternatively, curing can be carried out in a static moulding press. In that case, the first curing stage takes place with the press open with the material resting on the lower heated moulding plate. The second curing stage takes place with the press closed and both surfaces of the material under pressure and in contact with both upper and lower heated moulding plates. Each natural fibre mat can be considered to have a first face (A face) and a second face (B face). Preferably, the first bio-resin is applied to the first face of the natural fibre mat to at least partly impregnate the first face of the natural fibre mat with the first bio-resin. The second bio-resin may be applied to the second face of the natural fibre mat and/or to the core. During the curing step, therefore, it is the first bio-resin which is closest to a heat source (typically in the form of a heated mould). Therefore the first bio-resin "sees" a higher temperature than the second bio-resin. The first and second bio-resins have different roles. The role of the first bio-resin is primarily as a binder for the natural fibre mat, and controls the surface finish of the final composite panel. The role of the second bio-resin is primarily to bond the skin to the core.

Therefore it is preferred that the second bio-resin differs from the first bio-resin in terms of composition, quantity, curing temperature, curing speed, viscosity and/or water content. Of these, it is particularly preferred that the second bio-resin cures faster and/or at a lower temperature than the first bio-resin. The application of the resin to the respective faces of the natural fibre mat may be achieved through different processes. For example, the first resin may be impregnated by full immersion of the natural fibre mat in a bath of the first resin. This is preferred when a complete covering of the natural fibres in the mat with the first resin is required. Alternatively, one face may be contact coated, e.g. using a roller coating process. The other side may be sprayed. Alternative coating processes include blade coating, sprinkling, painting, dipping, etc. The purpose of using different application processes relates to the different bio-resins having different physical properties (e.g. different viscosity) and so the different bio-resins are most efficiently applied to the mat using different application processes. Furthermore, the different application processes allows different quantities of resin to be applied to each face.

The characteristics of the final product can be varied considerably according to need by varying the distribution of the resin through the natural fibre mat, its thickness and the pressure applied.

The process of impregnation of the resin into the natural fibre mat can be in the form of a continuous process. In that case, the width of the mat is limited only by the width of the coating machinery. Furthermore, the manufacture of the board itself (including the pressing and curing step) can be part of the continuous process. The resulting sheet of board may then be guillotined to size as it emerges from the production machinery. Alternatively, the resin-impregnated natural fibre mat can be used as a prepreg, the prepreg being cut to length as desired, applied to the core and then pressed and cured in a static press.

The thickness of the core is preferably at least 5mm or at least 10mm. The thickness of each skin layer (after the curing step) is preferably at least 5 times smaller than the thickness of the core. For each skin layer, the natural fibres are preferably arranged substantially randomly but substantially parallel to each face of the skin layer. The natural fibres may have an average length of at least 10mm, more preferably at least 20mm, at least 30mm, at least 40mm, at least 50mm, at least 60mm or at least 70mm. The fibres may be processed (e.g. cut) to have a maximum length of up to 150mm, for example. 6

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Preferably, the natural fibres are plant-derived fibres. Preferably, the plant-derived fibres are one or more selected from the following: hemp, jute, flax, ramie, kenaf, rattan, soya bean fibre, okra fibre, cotton, vine fibre, peat fibre, kapok fibre, sisal fibre, banana fibre or other similar types of bast fibre material. Such fibres are considered to be annually renewable, in that they are based on a crop which can be grown, harvested and renewed annually.

Preferably, the natural fibre mat is a non-woven mat formed by needle punching.

Alternatively, air laying may be used. Non-woven mats are preferred. However, woven mats may be used. The area density of the natural fibre mat may be in the range 300- 3000 grams per square metre (gsm).

Preferably, the natural fibre mat is dried before use in the process. A natural fibre mat with moisture content of less than 5wt% is suitable, e.g. about 3wt%.

The temperature applied in order to cure the first and second resin-impregnated mats is typically at least 130°C. Preferably, the temperature is 250°C or lower, more preferably 200°C or lower. A typical applied temperature for curing is 150-190°C. As discussed above, the curing process for the panel also includes pressing. By the combination of heating and pressing, the impregnated natural fibre mats are typically permanently compressed (preferably to different degrees) in a thermoset reaction so that the density of the skin layers after heating and pressing may be at least three times (preferably at least four times or at least five times) the density of the impregnated natural fibre mat before heating and pressing.

The bio-resin may be derived from sugar cane. Preferably, the bio-resin comprises furfural (furan-2-carbaldehyde) or a derivative of furfural such as furfural alcohol, furan, tetrahydrofuran and tertahydrofurfural alcohol (collectively referred to as furans). In particular, it is preferred that the bio-resin is a furan resin, such as a resin comprising prepolymers of furfuryl alcohol. The cured resin may therefore be a poly(furfuryl alcohol).

For example, a furan resin may be produced in which furfural replaces formaldehyde in a conventional production of a phenolic resin. The furan resin cross links (cures) in the presence of a strong acid catalyst via condensation reactions. Furfural is an aromatic aldehyde, and is derived from pentose (C5) sugars, and is obtainable from a variety of agricultural byproducts. It is typically synthesized by the acid hydrolysis and steam distillation of agricultural byproducts such as corn cobs, rice hulls, oat hulls and sugar cane bagasse. Further details relating to furan resins whose use is contemplated in the present invention is set out in "Handbook of Thermoset Plastics", edited by Sidney H. Goodman, Edition 2, Published by William Andrew, 1998, ISBN 0815514212,

9780815514213, Chapter 3: Amino and Furan Resins, by Christopher C. Ibeh, the content of which is incorporated herein by reference in its entirety.

Furan resins are of particular interest because they are derived from natural, renewable sources, they bond well to natural fibres and they have good flame-retardancy properties.

The bio-resin preferably includes an acid catalyst. The catalyst promotes curing via condensation reactions, releasing water vapour. The bio-resin may further include a blocker component. The function of the blocker component is to affect the curing behaviour of the bio-resin. Thus, the first and second bio-resins may differ in

composition, based on the content and/or type of catalyst and/or the content and/or type of blocker component. This allows the second bio-resin to cure faster and/or at a lower temperature than the first bio-resin component. The present inventors have realised in particular that if the bio-resins applied to the first and second faces of the mat are identical, then during the curing step in which the wet sandwich is heated and pressed, the bio-resin at the first face of the natural fibre mat will cure first, in view of the temperature gradient across the wet sandwich. However, the cured bio-resin can form a thermal barrier, thus reducing the flow of heat to the second face of the mat for curing of the bio-resin at that location. This results either in incomplete curing of the bio-resin at the second face or in an unacceptably long curing time for the panel. Without wishing to be bound by theory, the present inventors therefore consider that promoting curing of the bio-resin at the second face at a lower temperature and/or at a faster rate than the bio-resin at the first face can lead to significantly improved properties of the cured panel.

References in this disclosure to the second bio-resin cures faster and/or at a lower temperature than the first bio-resin relate to analysis of the respective bio-resins under identical conditions when subjected to heating up to their respective curing temperatures at the same rate of heating. It will be understood that the behaviour of the first and second bio-resins in situ in the composite depends heavily on the thermal gradient across the composite. However, in situ during the process of the present invention, it is preferred that the second bio-resin associated with one skin layer is cured before the first bio-resin associated with the same skin layer is completely cured. The present inventors have also arrived at an important realisation in terms of moisture management during the curing process. Curing of the bio-resin typically takes place via condensation reactions and therefore releases water vapour. Since the core of the panel is typically formed of paper or card, release of large quantities of water vapour into the core can cause unacceptable reductions in strength of the core. Furthermore, it is considered that if the bio-resin at the first face of the natural fibre mat is allowed to cure before the bio-resin at the second face, then moisture generated during curing of the second face may become trapped in the core, leading to weakening of the core. Again, the inventors make these observations without wishing to be bound by theory. During the curing step, preferably steam release means are provided. The curing step typically takes place in a heated moulding press. Known moulding presses are known for moulding polyurethane-containing products. Such moulding presses are typically operated in a carefully sealed condition, in view of the health and safety issues surrounding the curing of polyurethane. However, for the panels of the present invention, it is preferred instead that a steam vent is provided. The steam vent may take the form of an additional process step, in which the mould press is opened (at least partially) during the curing step in order to release steam that has been generated during the heating and curing of the panel. In that case, the mould press is typically then closed again to complete the curing of the panel. The steam vent may take place at 1 minute or less from the closure of the moulding press, more preferably at 45 seconds or less from the closure of the moulding press. Additionally or alternatively, the steam vent may take the form of a gap provided in the mould press throughout the curing step, the gap being placed so as to provide a suitable exit route for steam generated during the heating and curing of the panel.

It is possible to consider the cycle time of the manufacture of a panel according to the present invention. The cycle time is considered to be the time between corresponding steps in the manufacture of a first panel and a subsequent panel in the same moulding apparatus. Preferably, the cycle time here is 150 seconds or less. More preferably, the cycle time is 120 second or less, e.g. about 100 seconds. Preferably, the viscosity of the first bio-resin at room temperature is lower than that of the second bio-resin under the same conditions. This allows the first bio-resin to penetrate more deeply into the natural fibre mat. The second bio-resin in contrast penetrates less deeply and so is available for providing a secure bond between the core and the skin layer. In order to provide this difference in viscosity, the first and second bio-resins, at the time of application to the natural fibre mat, may differ in water content. It is preferred that the first bio-resin has a higher water content than the second bio-resin. In view of this higher water content, the partially impregnated natural fibre mat may be subjected to a drying step after application of the first bio-resin and before the application of the second bio-resin. The water content of the second bio-resin is preferably 10wt% or less. The water content of the first bio-resin (after an optional drying step of the impregnated mat) is preferably 10wt% or less. However, the water content of the first bio-resin at the time of impregnation into the first face of the natural fibre mat may be higher than 10wt%, e.g. 15wt% or higher, or 20wt% or higher, e.g. about 25wt%.

The first bio-resin preferably also includes a mould-release agent. This is preferred in order to assist in release of the cured panel from a corresponding mould. Preferably, the second bio-resin does not include a mould-release agent, since it is not needed at the interface between the paper core and the skin layer. Inclusion of a mould-release agent in the second bio-resin would likely have the effect of deleteriously reducing the strength of the panel.

The first bio-resin preferably also includes a filler component. The filler component can provide greater structural integrity to the free faces of the panel after curing. The filler is preferably a particulate filler such as a quarte-based powder. The average particle size of the filler may be less than 100pm, e.g. about 30μπι. The first bio-resin may include up to about 30wt% filler, more preferably up to about 20wt% filler.

Preferably, different amounts by weight of the first and second bio-resins are applied in the manufacture of the composite panel. Typically, more of the first bio-resin is applied than the second bio-resin. For example, 1.5-3 times (or more, e.g. 1 .5-10 times) by weight the amount of the first bio-resin may be applied (to the mat) than the second bio- resin (to the mat and/or to the core). Typically, the second bio-resin may be applied in an amount of 100-200gm^"2. Typically, the first bio-resin may be applied in an amount of 200-1 OOOgm^"2. As indicated earlier, the first bio-resin may be fully impregnated into the mat. The basis weight of the natural fibre mat is preferably in the range 400-800gm^"2.

The natural fibre mat may include continuous reinforcing yarn laid at or close to the surface of the mat. The reinforcing yarn preferably runs across the width and/or length of the mat. This can provide substantial reinforcement to the panel, further improving the mechanical properties of the panel. The reinforcing yarn should be continuous and thus may be synthetic. However, by weight the proportion of continuous reinforcing yarn in the finished panel is typically very low.

Further optional features of the invention are set out below.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

Fig. 1 illustrates a typical natural fibre mat manufacturing process.

Fig. 2 shows a schematic bio-resin coating and drying plant according to an embodiment of the invention.

Fig. 3 shows a modified schematic bio-resin coating and drying plant according to an embodiment of the invention.

Fig. 4 shows another modified schematic bio-resin coating and drying plant according to an embodiment of the invention.

Fig. 5 shows a schematic moulding process for moulding a panel according to an embodiment of the invention.

Figs. 6 and 7 show schematic views of impregnated natural fibre mats for use with embodiments of the invention.

Figs. 8 and 9 show schematic cross sectional view of composite panels according to embodiments of the invention.

Fig. 10 shows a typical layer of bio-resin impregnated natural fibre mat for use in an embodiment of the invention.

Fig. 11 shows a schematic view of a manufacturing process according to an embodiment of the invention.

Fig. 12 shows a schematic cross sectional view of a composite panel according to an embodiment of the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS. AND FURTHER OPTIONAL FEATURES OF THE INVENTION

Fig.1 illustrates a production line for the preparation of a natural fibre mat 7 (hereinafter called "the mat"), wherein natural fibre is mixed in a mixer 1 and dispensed from a hopper 2 on to a chute feed conveyor 3. The fibres are aligned in a carding machine 4 and passed through a cross lapper 5. The cross lapped layers are then consolidated in a needle punch machine 6. The resulting mat 7 is then rolled up for further treatment or passed directly to a resin coating plant. The basis weight of the natural fibre mat is typically in the range 400-800gm^"2, e.g. 600gm^"2.

It is mentioned here that reinforcing yarn may be included in the mat, at or near the surface of the mat. The reinforcing yarn my be incorporated into the mat during the cross-lapping process. In the process, a stack of offset overlying layers of natural fibre web are laid by cross-lapping and the reinforcing yarn is added to each layer at the same time as, or immediately before, the layer is laid. The reinforcing yarn is provided only at or near one or more edge of the layer, so that after cross-lapping, the reinforcing yarn is located only at positions at or near to one or more surfaces of the stack. Further detail relating to this process is set out in International patent application PCT/GB201 1/001472 (unpublished at the time of writing this disclosure), the content of which is hereby incorporated by reference in its entirety.

Fig.2 illustrates a typical scheme 8 for the production of resin-coated mat having one side of the fibre mat coated with bio-resin. Untreated mat 7 is unwound and passed over a chromed roller 9 which forms one side of a bio-resin bath 0 containing a first bio-resin. Rear blade 10a defines the rearward extent of the bio-resin bath. The first bio-resin (resin A) is thereby applied to the first face (face A) of the mat. A J-profile scraper blade 10b is used to press bio-resin A into the mat and to ensure uniformity of loading of bio- resin A in face A.

The one-side-coated mat 11 is then passed through a pair of calendar rollers 12 and is dried in an oven 13 in order to reduce the water content of the one-side-coated mat 1 1. In the process shown in Fig. 2, the one-side coated mat is then rolled up into a roll 14. The one-side coated mat may then be treated in order that the second face (face B) of the mat is coated with a second bio-resin (resin B). This may be carried out, for example, using a roller-coater or spray coater 14. The second bio-resin differs from the first bio- resin as explained in more detail below. The weight (per unit area) of bio-resin applied to the second face is lower than the weight (per unit area) of bio-resin applied to the first face. It should be noted that in alternative embodiments, described below, resin B may instead be applied directly to the core.

In modified embodiments of the process, resins A and B can be applied to the different faces of the mat in a single process. For example, Fig. 3 shows a schematic view of the modified process. Dry natural fibre mat 20 is unwound from roll 22 and passed over chromed roller 24. Face B of the mat is in contact with the roller 24 and face A is exposed to resin bath 26 which is defined by rear blade 28, front J-scraper blade 30 and the roller 24. In this way, resin A is impregnated into the A face of the mat. The one-side coated mat is then passed through oven 32 to reduce the water content of resin A. On exit from the oven, release paper 34 from release paper roll 36 may be applied over the A face of the one-side coated mat. The mat is then inverted using a series of rollers 36 so that face B of the mat faces upwardly. Resin B is then sprayed onto face B using sprayer 38. The coated mat is then rolled up on roll 40.

A further modified process is illustrated in Fig. 4. The initial stages of the process are similar to Fig. 2 and so are not described again here. On exit from drying oven 50, the one-side coated mat reaches double roller coater 52. Upper roller 54 acts in opposition to lower roller 56. A reservoir 60 of resin B is maintained between lower roller 56 and auxiliary roller 58. Reservoir 60 ensures a consistent thickness of resin B is maintained on lower roller 56, and so ensures a consistent application of resin B to the B face of the mat. The impregnated mat 62 is then wound on roll 64.

The resulting resin coated mat can then be transported or taken directly to a moulding press.

It is possible for the resin-impregnated mat to be pressed and heated in order to form a cured board. However, in the preferred embodiment of the present invention, a paper or card honeycomb core is placed between two of the resin-impregnated mats to form a sandwich construction. For each mat, face B is placed in contact with the core and face A is the outward face of the sandwich construction. The sandwich construction is then pressed and heated in order to cure the bio-resin, compress the mats to form skins and achieve a bond between the core and the skins. The sequential pressing and heating process is described in more detail below in order to provide the skins with different acoustic properties.

Fig. 5 shows a schematic moulding process for moulding a panel according to an embodiment of the invention. Paper honeycomb core 70 is provided and two rolls 72, 74 of resin-impregnated natural fibre mat as described above are also provided. The upper layer 72 of impregnated mat is contacted with the core (optionally coated with resin B) with the B face in contact with the core and the A face uppermost. The lower layer 74 of impregnated mat is contacted with the core with the B face in contact with the core (optionally coated with resin B) and the A face lowermost. A wet sandwich construction is therefore provided. This can be cut to shape as required, e.g. by guillotine.

The wet sandwich is then placed in a moulding press 76 having upper 78 and lower 80 mould plates. Each mould plate includes a series of gaps 82 which allow steam to be vented during the curing step.

The moulding press illustrated in Fig. 5 is a static moulding press in the sense that the sandwich construction remains in the same position during the curing step. Upper 78 and lower 80 mould plates can be heated independently. In a first curing stage, the sandwich construction is placed between the mould plates with the lower mould plate 80 heated to a temperature suitable for curing resin A. The upper mould plate 78 is at a lower temperature. The mould plates 78, 80 are brought towards each other to press the sandwich construction relatively gently. Resin A in the lower mat begins to cure, but the mat is not highly compressed and so the resulting cured (or partially cured) structure is relatively porous. The second curing stage then takes place, in which the first mould plate 78 increases in temperature to a temperature suitable for curing resin A and the separation between the mould plates is decreased in order to press the sandwich construction more firmly. Resin A in the upper mat is then cured, but since the pressure in this stage is greater, the resultant cured structure is relatively non-porous.

In a modified embodiment, upper 78 and lower 80 mould plates may be heated to the same temperature for each stage of the curing step, but in the first curing step, the moulding press may be kept open for the first curing stage And then closed for the second curing stage. Fig. 10 shows a typical layer of bio-resin impregnated natural fibre mat for use in an embodiment of the invention. A resin-impregnated natural fibre mat is shown in which the natural fibre material 200 is impregnated by coating it at face A with resin 202 (resin A). Depending on the consistency and viscosity of resin 202, it is allowed to fully or partially impregnate the natural fibre material 200. In some applications the mat is coated on both faces with resin A, or may be may be fully-impregnated with resin A before a layer of resin B is applied to face B (the face that will be facing the core).

Alternatively the layer of resin B may be applied to the core material. Fig. 11 shows a schematic view of a manufacturing process according to a preferred embodiment of the invention. The manufacturing of the acoustic panel is achieved here in a sequential process using a machine such as a double belt press. In a production line which progresses in the direction of the arrow 210, layers of uncured resin-impregnated fibre mat 212 are applied to both sides of a core 214 of sound absorbing material such as paper honeycomb. This sandwich is passed into a series of belt rollers to compress and cure the layers. In the first stage one belt 216 is hot and the opposing belts 218 are cold and are arranged to give progressive compression to the layered materials.

As the layered materials emerge from the belt 218 the lower impregnated mat 212 is partially cured but remains uncured at its interface with the core material 214. The upper impregnated mat 212 is not cured.

In the next stage of the process, the layered materials pass between two heated roller belts 216 and 220 which fully cure both sides under pressure. An increase in pressure is achieved by a reduction in the gap between the two belts between the first and second stages. The final stage is to pass the laminated board 222 through cold roller belts 224 where it is cooled and held flat under compression. The result of this process is that in the laminated board, the lower skin of the composite has a lower density than the upper skin.

In a variant of the process similar results can be achieved using a static press as described above or a series of static presses such that discrete sheets of material are pressed and heat-cured sequentially as specified in the continuous production line process described above. 46

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Fig. 12 shows a schematic cross sectional view of a composite panel according to an embodiment of the invention. Core 230 has upper 232 and lower 234 skin layers. Upper layer 232 is a hard, relatively thin layer of resin impregnated fibre board which is substantially acoustically reflective and is less porous to sound than the other lower thicker skin layer 234 of impregnated fibre board which is acoustically porous or semi- porous.

The whole has excellent stiffness and thermal insulation properties as well as acoustic characteristics which can be widely varied by adjusting the thicknesses of the various materials, the compression applied and the phased curing times and temperatures.

Bio-resins A and B are each based on furan resin. Furan resin can be obtained, for example, from TransFurans Chemicals bvba, Industriepark Leukaard 2, 2440 GEEL, Belgium, under the trade name BioRez™, for example BioRez™ 050525 S 1B. Each bio-resin is cured via a heat-activated catalyst. The curing temperature of this resin is in the range 140-190 degrees C. Storage of this product at 20-25 degrees C is possible for extended periods, e.g. up to one month or longer. A suitable catalyst is maleic acid.

This is preferred since it is considered not to have an adverse effect on the natural fibres of the mat. An alternative catalyst is citric acid. Suitable furan resin can be obtained by the following process. Hemicellulosic agricultural waste (e.g. waste from sugar cane) are rich in pentose sugars. Controlled hydrolysis of pentose sugars gives furfural. This is converted to furfuryl alcohol by catalytic hydrogenation. Furfuryl alcohol can then be formed into prepolymers of furfuryl alcohol, which is the base material of the resin. One particular advantage of furan resin is that it is substantially free from volatile organic solvents. Furthermore, during the curing process, water is formed from condensation reactions which occur as the resin crosslinks.

Bio-resin A has a lower viscosity at room temperature than bio-resin B. This allows bio- resin A to penetrate more deeply into the mat. In the preferred embodiment, bio-resin A is fully impregnated into the mat. Bio-resin B in contrast penetrates less deeply and so is available for providing a secure bond between the core and the skin layer. At the time of its application to the mat, bio-resin A has a higher water content than bio-resin B, this contributing to the lower viscosity. It is in view of this higher water content, the partially impregnated mat is subjected to a drying step as shown in Fig. 2. After the drying step, bio-resin A has a water content of 10wt% or less. . Bio-resin B has a water content of 10wt% or less, and so there is no need for a further drying step after application of bio-resin B to face B of the mat. In the preferred embodiment, bio-resin B is applied directly to the core.

Bio-resin A may include a mould-release agent, but bio-resin B does not. The mould- release agent is Diamond Coat 3551 BX.

Bio-resin A may include about 20wt% of a filler component, but bio-resin B does not. The filler is quarz powder with average particle size of about 30pm. A suitable filler is quartz flour (Microsil® M) available from Sibelco Benelux.

Bio-resin A is applied to face A of the mat at about 250-1 OOOgm^"2. Bio-resin B is applied to face B of the mat or directly to the core at about 100-150gm^"2.

Suitable curing temperatures for resins A and B are as follows. Resin A cures at 170°C. Resin B cures at 140° C.

Suitable resins for use as resins A and B are BioRez™ 050525 and BioRez™ 080101 , respectively, available from TransFurans Chemicals bvba. During the curing step, steam is produced from the condensation reactions taking pace in the bio-resin and also due to further drying of the mats and the resin. The moulding press may be opened briefly shortly after the beginning of the curing step in order to vent steam. This may take place after 1 minute of the curing step, but preferably it takes place after about 30 seconds of the curing step. The moulding press is then closed in order to continue with the curing step. Alternatively, as shown in Fig. 5, steam vent openings may be provided in the moulding press in order to release steam throughout the curing step.

Figs. 6 and 7 show schematic views of impregnated natural fibre mats. In Fig. 6, natural fibre mat 100 has resin A 102 coated and impregnated at the A face and resin B 104 coated and impregnated at the B face. The loading of resins A and B is substantially equal. In contrast, in Fig. 7, natural fibre mat 106 has resin A 108 coated and impregnated at the A face and resin B 110 coated and impregnated at the B face. The loading of resin A is significantly greater than the loading of resin B. Figs. 8 and 9 show schematic cross sectional views of composite panels according to embodiments of the invention. Outer skins 120 and 122 adhere to a core 124 and 126 of paper or card honeycomb. Fig. 8 illustrates the use of fibre mat corresponding to Fig. 6 having a similar weight loading of bio-resin on face A and face B of the mats. Fig. 9 illustrates the use of fibre mat corresponding to Fig. 7 having a greater weight loading of bio-resin on face A than on face B of the mats.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

All references referred to above are hereby incorporated by reference.

Claims

1. An acoustic panel having a sandwich construction with a core formed of acoustically absorbent material and first and second skin layers comprising natural fibre and cured bio-resin, wherein the first and second skin layers have substantially different acoustic properties.

2. A panel according to claim 1 wherein the core is formed of a paper or card cellular structure.

3. A panel according to claim 1 or claim 2 wherein the first skin layer has a lower density than the second skin layer.

4. A method for manufacturing an acoustic panel having a sandwich construction with a paper or card cellular core and first and second skin layers comprising natural fibre and cured bio-resin, wherein the method includes the steps:

(i) providing first and second natural fibre mats;

(iii) providing a core formed of acoustically absorbent material;

(v) applying the first and second impregnated natural fibre mat to opposing sides of the core, thereby forming a wet sandwich construction; and

(vi) pressing and heating the wet sandwich construction so as to compress the impregnated natural fibre mats and cure the first and second bio-resins to form the first and second skin layers,

5. A method according to claim 4 wherein first and second skin layers have substantially different acoustic properties as a result of different applied pressure during curing.

6. A method according to claim 4 or claim 5 wherein the pressure applied during curing of the first resin-impregnated mat is lower than the pressure applied during curing of the second resin-impregnated mat.

7. A method according to claim 6 wherein, in a first curing stage, the first resin- impregnated mat is subjected to a higher temperature than the second resin-impregnated mat so that the first resin in the first resin-impregnated mat begins to cure whilst the first resin in the second resin-impregnated mat does not, and in a second curing stage, the second resin-impregnated mat is subjected to a higher temperature than in the first curing stage in order to cure the first resin in the second resin-impregnated mat, the pressure applied in the second curing stage being greater than in the first curing stage.

8. A method according to any one of claims 4 to 7 wherein curing is carried out in a sequential moulding apparatus.

9. A method according to any one of claims 4 to 8 wherein the second bio-resin differs from the first bio-resin in terms of composition, quantity, curing temperature, curing speed, viscosity and/or water content.

10. A method according to any one of claims 4 to 9 wherein the first resin is impregnated into the natural fibre mat by full immersion of the natural fibre mat in a bath of the first resin.

11. A method according to any one of claims 4 to 10 wherein each bio-resin is a furan resin.