Expanded cellular foam core and manufacture thereof

Abstract

A core material for a sandwich panel comprising a core layer of thermoplastic polymer expanded cellular foam. The layer comprising first and second surfaces separated by the thickness of the core, th

Description

[001] The present invention relates to a core material for a sandwich panel, the core material comprising a core layer of expanded cellular foam, and to a method of manufacturing such a core material for a sandwich panel. The core layer is composed of a thermoplastic polymer, for example polyethylene terephthalate (PET).

[002] It is well known to those skilled in the art of manufacturing sandwich panels to provide a core layer of an expanded cellular foam composed of a thermoplastic polymer. The core layer is laid up between opposing fibrous layers and the fibrous layers are provided with a polymer resin to form fibre reinforced resin matrix composite outer layers on opposite sides of the core layer.

[003] When manufacturing large structural components, such as wind turbine blades, the core layer of expanded cellular foam is disposed between dry fibrous layers, and then liquid resin, which is a thermosetting resin such as an epoxy resin, or a thermoplastic resin, is infused into the dry fibrous layers. Typically, the liquid resin infused is by a resin transfer moulding (RTM) process, such as a vacuum assisted resin transfer moulding process (VARTM), which processes are generally known to those skilled in the art, in which the structural component is manufactured in a mould.

[004] The structural component to be manufactured typically has a three dimensional morphology, including curvature in at least one outer surface. Accordingly, the sandwich panel needs to include such curvature, and in particular the core material must be flexible in order to be able to be flexed to provide a curvature matching the curvature of the mould surface on which the layers of the sandwich panel are laid up prior to the moulding process.

[005] In order to provide a core layer with the desired flexibility, two known core structures are typically employed. In a first core structure, a core material comprises an assembly of individual segments formed into a core layer and on one side of the core layer a flexible fabric material is attached; the core material can be flexed by bending the fabric material at the junctions between the adjacent segments. In a second core structure, a core material comprises a unitary core layer which has a series of closed end cuts extending into the core layer from one or both sides. The cuts are sufficiently deep so that the remaining material of the core layer, which is between the closed end of the cut and the opposite side of the core layer, can function as a hinge, so that the core material can be flexed about the hinges.

[006] One problem with these known core structures is that during the moulding process, liquid resin can infuse into exposed cells of the expanded cellular foam, which causes undesirable resin uptake of the foam core. Such resin uptake increases the weight of the resultant moulded structural component but does not improve, and in contrast may impair, the required mechanical properties of the structural component.

[007] Another problem with these known core structures is that in some cores, in order to reduce resin uptake, the core is designed with a lower exposed surface area, by increasing the dimensions of individual segments or reducing the area of the cuts dividing the foam into segments, which can reduce the flexibility of the foam core.

[008] It is already known, for example from US2015 / 125686A1, to produce foam cores in which the opposite outer surfaces have been subjected to a heat-sealing process in order to reduce resin uptake at those surfaces, and to produce foam cores by welding, also called fusing, together individual foam elements. However, this process does not provide the desired flexibility in the foam cores. When the core is cut to provide grooves to provide flexibility and increased resin infusion performance, the cells of the cellular foam are cut and exposed, which increases resin uptake, which is undesirable as explained above.

[009] US2023 / 294377A1 discloses a foam core layer sub-divided, except for a connecting layer, into a plurality of body elements. A surface layer of the connecting layer and the adjoining surface layer of the body elements have a thermally densified layer formed by hot wire cutting which seals the pores of the foam. Although the provision of the body elements provides flexibility and the thermally densified layer partly seals the foam core on one surface, nevertheless the foam core still exhibits undesired resin uptake at foam surfaces other than the thermally densified layer.

[010] Accordingly, there is a need in the art for a core material, and a manufacturing method therefore, which exhibits the combination of (i) low resin uptake and (ii) high flexibility.

[011] The present invention aims to meet this need in the art and to overcome the problem of achieving the combination of (i) low resin uptake and (ii) high flexibility in a core material comprising a core layer of expanded cellular foam composed of a thermoplastic polymer.

[012] In accordance with a first aspect of the present invention, there is provided a core material for a sandwich panel according to claim 1.

[013] In accordance with a second aspect of the present invention, there is provided a method of manufacturing a core material for a sandwich panel according to claim 13.

[014] Further preferred features of these various aspects of the present invention are defined in the respective dependent claims.

[015] The present invention is predicated on the finding by the present inventors that structural features can be incorporated into a core layer of expanded cellular foam which not only increase the flexibility of the core layer but also do not provide additional exposed surfaces through which liquid resin could pass into the expanded cellular foam during liquid resin infusion which would otherwise increase the resin uptake of the core layer.

[016] In particular, the present inventors have found that foam surfaces, extending partially into or entirely through the thickness of the core layer, that form dividing regions between adjacent segments in the core layer, can be sealed against liquid resin ingress into the expanded cellular foam and yet can provide a series of regions at which the foam core can be flexed about an axis to provide a curvature in the morphology of the core layer.

[017] The provision of these sealed internal surfaces which impart flexibility to the foam core layer can be achieved, in general, by three alternative structures.

[018] In a first structure, the core layer is provided with a series of dividing regions between adjacent segments in which each dividing region comprises opposed sealed surfaces, and the dividing region has a closed end to define an integral hinge within the core layer.

[019] The integral hinges provide the desired flexibility and the sealed surfaces reduce resin uptake.

[020] Furthermore, the opposed sealed surfaces of each dividing region are in contact, which further reduces resin uptake because any gap between the opposed sealed surfaces is either minimised or avoided.

[021] Still further, by providing that the opposed sealed surfaces of each dividing region are in contact, minimal resin flows between the opposed sealed surfaces, which provides that in the resultant sandwich panel, the core density over the surface area of the core is more uniform in flat geometries.

[022] When the opposite outer surfaces of the core layer are also sealed, substantially the entire surface of the core layer which is exposed to liquid resin during the moulding process is sealed against liquid ingress into the expanded cellular foam. Accordingly, resin uptake is minimised, while providing the desired flexibility to the core layer.

[023] Furthermore, the provision of integral hinges avoids the requirement for an additional fabric layer, for example a scrim layer, to be attached to the core layer to provide the flexibility functionality. A scrim-less core layer comprised solely of a single thermoplastic polymer foam, for example PET, has reduced cost and is easier to recycle as compared to a core layer incorporating an additional scrim material.

[024] In a second structure, the core layer is provided with a series of dividing regions between adjacent segments in which each dividing region comprises opposed sealed surfaces, and the dividing region has a closed end to define an integral hinge within the core layer. The integral hinges provide the desired flexibility and the sealed surfaces reduce resin uptake. The opposite sealed surfaces in each dividing region are weakly bonded together by a fusion-bonded connection (i.e. a weld formed by joining together two opposed molten surfaces composed of the thermoplastic polymer). The fusion-bonded connection can easily be manually snapped at any desired dividing region so that at that dividing region the core layer can be flexed about the associated integral hinge. Other fusion-bonded connections can be left unsnapped at any desired dividing regions at which the core layer is required to be in a planar configuration.

[025] As compared to the first structure, the provision of fusion-bonded connections can further reduce resin uptake without reducing the desired flexibility of the core layer. This is because at regions where the core layer can remain planar, the fusion-bonded connections can remain in place, and unsnapped, so that no resin infusion into any gap between the sealed surfaces, or possibly by leakage through the sealed surfaces, can occur.

[026] Embodiments of the present invention will now be described by way of example only, with reference to the accompanying drawings, in which: Figure lisa schematic perspective view of a core material for a sandwich panel in accordance with a first embodiment of the present invention; Figure 2 is an optical photomicrograph of a portion of opposing side surfaces of a dividing region between adjacent segments of the core material of Figure 1; Figure 3 is a photograph showing a perspective view of a specific core material of the first embodiment of the present invention in a planar configuration; Figure 4 is a photograph showing a perspective view of the specific core material of Figure 3 in a curved configuration; Figure 5 is an optical photomicrograph showing a side view of a hinge in the specific core material of Figure 3 when in a curved configuration; Figure 6 is a schematic side view of a portion of a core material for a sandwich panel in accordance with a second embodiment of the present invention in a planar configuration; Figure 7 is a schematic side view of the portion of the core material shown in Figure 6, which is in a curved configuration; Figure 8 is a schematic side view of a portion of a core material for a sandwich panel, which is in a planar configuration, in accordance with a third embodiment of the present invention; Figure 9 is a schematic side view of the portion of the core material shown in Figure 8, which is in a curved configuration; Figure 10 schematically illustrates the process flow of a method for manufacturing core material for a sandwich panel in accordance with a further embodiment of the present invention; Figure 11 schematically illustrates an alternative apparatus for sealing the surface of a core layer for manufacturing core material for a sandwich panel in accordance with a further embodiment of the present invention; Figure 12 schematically illustrates part of a core layer produced using the apparatus of Figure 11; Figure 13 shows a process flow, including two alternative embodiments, for producing a core layer illustrated in Figure 12; and Figures 14 (a) to 14(d) show the relationship between the resin uptake and core thickness for a variety of different core thicknesses and radii of curvature for core layers in accordance with Examples of the present invention and Comparative Examples representing conventional core kitting including cut grooves.

[027] Referring to Figure 1, there is schematically illustrated a core material 4 for a sandwich panel in accordance with a first embodiment of the present invention. The core material 4 is shown in an unloaded state, in a planar configuration.

[028] The core material 4 comprises a core layer 6 of expanded cellular foam 8 composed of a thermoplastic polymer. The thermoplastic polymer may comprise any polymer suitable for use, or known for use by those skilled in the art of manufacturing sandwich panels, as a core layer. Typically, the thermoplastic polymer comprises a polyester, for example a polyalkylene terephthalate. Preferably, the thermoplastic polymer comprises polyethylene terephthalate (PET). Typically, the thermoplastic polymer comprises polyethylene terephthalate and the expanded cellular foam has a density of from 60 to 300 kg / m3.

[029] The polyethylene terephthalate used to make the expanded cellular foam 8 may be made from virgin PET (vPET) or recycled PET (rPET), or a mixture thereof. The manufacture of expanded cellular foam composed of PET, for example in the form of extruded panels which may then be cut, shaped and bonded together to form various multi-element configurations, is, in general, well known to those skilled in the art.

[030] The core layer 6 has opposite first and second surfaces 10, 12. A thickness T of the core layer 6 extends between the first and second surfaces 10, 12. Typically, the core layer has a thickness of from 5 to 80 mm.

[031] The core layer 6 is divided into a series of segments 14, shown as 14a, 14b, 14c, 14d, for four typical segments 14 in Figure 1, by a plurality of planar dividing regions 16, shown as 16a, 16b, 16c for the three typical dividing regions 16 shown in Figure 1.

[032] In accordance with the present invention, the series of segments 14 may comprise any desired number of segments 14, and a corresponding desired number of dividing regions 16. In the illustrated embodiment, for “n” segments 14 there are “n-1” dividing regions 16.

[033] The dividing regions 16 extend inwardly from the second surface 12 in a thickness direction Dt. The dividing regions 16 are oriented transversely to a longitudinal direction Dl of the core layer 6 which extends between opposite first and second ends 18, 20 of the core layer 6. Adjacent segments 14 are separated by a respective dividing region 16 at which planar opposing side surfaces 22, shown as 22a, 22b for two respective opposing side surfaces 22 in Figure 1, of the adjacent segments 16 are in mutual contact.

[034] In the embodiment of Figure 1, at each dividing region 16 the opposing side surfaces 22 of the adjacent segments 14 which are in mutual contact are free of any bonding therebetween.

[035] In the illustrated embodiment, the series is a linear series, i.e. the longitudinal direction Dtis a straight line. Also, in the illustrated embodiment, the dividing regions 16 are parallel. Furthermore, in the illustrated embodiment, the dividing regions 16 are orthogonal to the longitudinal direction Dl of the core layer 6. However, alternatively other configurations of the segments 14 and the dividing regions 16 in the core layer 6, for example non-parallel dividing regions 16 and dividing regions 16 inclined at an acute angle to the longitudinal axis, are within the scope of the present invention.

[036] The opposite first and second surfaces 10, 12, and the opposing side surfaces 22 of the adjacent segments 14 at each dividing region 16, comprise a non-cellular surface layer 24 of the thermoplastic polymer. The non-cellular surface layer 24 at least partially seals the expanded cellular foam 8 inwardly of the respective surface layer 24 from liquid ingress, in particular liquid resin ingress during liquid resin infusion, through the surface layer 24 into the expanded cellular foam 8. In Figure 1, the non-cellular surface layer 24 is illustrated highly schematically, and the depth dimensions are not to scale and are exaggerated for the purpose of clarity of illustration.

[037] Figure 1 illustrates respective non-cellular surface layers 24a, 24b at the first and second surfaces 10, 12, and respective non-cellular surface layers 24c, 24d at the opposing side surfaces 22 of the adjacent segments 14 at each dividing region 16. For clarity of illustration, the inner edges of the non-cellular surface layers 24 are shown by dashed lines in Figure 1.

[038] Each 16 dividing region terminates at a closed end 26 remote from the second surface 12. The non-cellular surface layer 24 at the first surface 10 extends between the first surface 10 and the closed end 26 to form a flexible hinge 28 between adjacent segments 14. Again, the distance of the closed end 26 from the first surface 10, to form the integral flexible hinge 28, is not to scale and is exaggerated for the purpose of clarity of illustration.

[039] The flexible hinge 28 has a thickness between the closed end 26 and the first surface 10 of from 0.1 to 3 mm, typically from 0.25 to 1 mm. Preferably, the flexible hinge 26 consists of the non-cellular surface layer 24 at the first surface 10.

[040] At each dividing region 16, the non-cellular surface layer 24, in the opposing side surfaces 22 of the adjacent segments 14, has a depth of from 0.05 to 1.5 mm, typically from 0.1 to 0.3 mm. In addition, at each dividing region 16 the non-cellular surface layer 24, in the opposing side surfaces 22 of the adjacent segments 14, has a gloss value of the surface measured at 60° in accordance with DIN 67530-1982 of from 1 to 10 gloss units, typically from 4 to 7 gloss units. In this specification, the “gloss units” are calibrated with reference to a highly polished reference black glass standard, having a refractive index of 1.567 for the Sodium D line, which has a specular reflectance of 100 gloss units at the specified angle.

[041] Figure 2 is an optical photomicrograph which shows a portion of the opposing side surfaces 22 in one particular example of this embodiment of the present invention. The photomicrograph is a cross-section through facing parts of two adjacent segments 14, and shows opposing non-cellular surface layers 24, each on a region of the expanded cellular foam 8. In Figure 2, the non-cellular surface layer 24 can be determined and the thickness of the non-cellular surface layer 24 is shown at a variety of different positions. In this Example, the measured thickness values range from a minimum of 69.8 pm to a maximum thickness of 223.34 pm, and the non-cellular surface layer 24 has an average thickness within the range of from 0.1 to 0.3 mm (i.e. 100 to 300 pm). It can be seen that the non-cellular surface layer 24 is a thin solid layer formed by fusion of the surface of the expanded cellular foam 8. The thin solid layer 24 is continuous and acts as a sealing layer on the internal surfaces 22 of the expanded cellular foam 8 formed in the dividing regions 16. It may be seen that the opposing side surfaces 22 are in contact at intermittent locations, in particular where relatively high points on the opposing surfaces are aligned.

[042] In addition, at each of the first and second surfaces 10, 12 the non-cellular surface layer 24 has a depth of from 0.05 to 1.5, typically from 0.1 to 0.3 mm. Also, at each of the first and second surfaces 10, 12 the non-cellular surface layer 24 has a gloss value of the surface measured at 60° in accordance with DIN 67530-1982 of from 1 to 10 gloss units, typically from 4 to 7 gloss units.

[043] The non-cellular surface layer 24 has a depth which is sufficient to form an integral hinge 28 but which does not significantly decrease the through-thickness tensile strength of the core layer 6.

[044] The non-cellular surface layer 24 on each of the first and second surfaces 10, 12 has a similar morphology to the individual non-cellular surface layers 24 shown in Figure 2.

[045] Figure 3 is a photograph of an Example of the core material 4 shown in Figure 1 in a planar configuration which is used for shipping and storage of the core material 4 prior to use in the manufacture of sandwich panels. In Figure 3 the second surface 12 of the core material 4 is uppermost. Figure 3 shows the adjacent segments 14 separated by the dividing regions 16.

[046] Figure 4 is a photograph of the core material 4 shown in Figure 3 in a non-planar configuration, and in Figure 4 the first surface 10 of the core material 4 is uppermost.

[047] During use, the core material 4 is subjected to bending about a curvature to match the three-dimensional shape of the core material 4 to the three-dimensional shape of one or more mould surfaces (not shown) for forming a sandwich panel.

[048] For manufacture of the sandwich panel, the core material 4 is located between opposing lower and upper fibrous layers prior to a moulding process, for example for forming a wind turbine blade. The core material is laid up into the mould, and if the mould surface has a three dimensional curvature about an axis, then the core material 4 needs to be flexed to match that curvature so that in the resultant moulded article, for example a wind turbine blade, the core material has a shape and configuration generally matching the desired shape and configuration of the outer surface of the moulded article which is substantially formed by the sandwich panel, and any associated paint and / or coating layers.

[049] As shown in Figure 4, by providing the series of flexible hinges 28, which are each aligned with a respective dividing region 16, the core layer 6 may be curved about an axis transverse to the longitudinal direction Dl by flexing the core layer 6 about one or more flexible hinges 28 thereby to separate adjacent segments 14 by the hinges 28 along respective dividing regions 16. The hinged separation of the segments 14 forms a gap 17 in the second surface 12 at the position of the dividing region 16, the gap 17 extending inwardly towards the flexible hinge 28 at the first surface 10.

[050] Figure 5 is an optical photomicrograph which shows a flexible hinge 28 after hinged separation of the adjacent segments 14 along the associated dividing region 16 in one particular example of this embodiment of the present invention. The photomicrograph is a cross-section through facing parts of two adjacent segments 14 after flexing, and shows the flexible hinge 28, which is formed by the non-cellular surface layer 24 at the first surface 10, both flexing and joining together the opposed segments 14. The gap 17 formed at the dividing region 16 is shown.

[051] In Figure 5, the thickness of the flexible hinge 28, and consequently the thickness of the non-cellular surface layer 24 which forms the flexible hinge 28, is shown. In this Example, the measured thickness of the flexible hinge 28 is 247.42 pm. The non-cellular surface layer 24 at the first surface 10 has an average thickness within the range of from 0.1 to 0.3 mm (i.e. 100 to 300 pm). It can be seen that the non-cellular surface layer 24 at the first surface 10 is a continuous thin solid layer formed by fusion of the surface of the expanded cellular foam 8. The continuous thin solid layer 24 acts as a flexible hinge 28 and also as a sealing layer on the external surfaces of the expanded cellular foam 8 formed on the first surface 10.

[052] When the core layer 6 is in the unloaded state, as shown in Figure 3, in some examples of core layers 6 of this embodiment, no spacing may be present between the adjacent segments 14 along the respective dividing regions 16, and the adjacent segments 14 may be in contact through the thickness of the core layer 6, and consequently the flexible hinge 28 has zero width in a direction parallel to the direction Dl. However, in other examples of core layers 6 of this embodiment, the flexible hinge 28 has a measurable width in a direction parallel to the direction Dl, to provide a spacing between the adjacent segments 14 along the respective dividing regions 16, and the adjacent segments 14 are mutually spaced through the thickness of the core layer 6 by a distance corresponding to the width of the flexible hinge 28. Typically, the flexible hinge 28 has a width in a direction parallel to the direction Dl of from 100 to 3000 microns, optionally from 200 to 500 microns.

[053] In the embodiment of Figures 1 to 5 described above, each of the opposite first and second surfaces 10, 12 comprises a non-cellular surface layer 24 of the thermoplastic polymer. However, in an alternative embodiment, only one of the opposite first and second surfaces 10, 12, i.e. the second surface 12 as described above, comprises a non-cellular surface layer 24 of the thermoplastic polymer, and at that one surface the non-cellular surface layer 24 forms an integral flexible hinge 28.

[054] Referring to Figure 6, there is schematically illustrated a section of a core material 104 for a sandwich panel in accordance with a second embodiment of the present invention. The core material is shown in an unloaded state, in a planar configuration. For clarity, again some dimensions are not to scale and are exaggerated for clarity of illustration.

[055] In this embodiment, the core material 104 has substantially the same general composition and structure as the core material 4 of the first embodiment, except that in the second embodiment at each dividing region 116 the opposing side surfaces 122 of the adjacent segments 114 which are in mutual contact are weakly bonded together by a fusion-bonded connection 130 between the opposing non-cellular surface layers 124, which as in the first embodiment are formed as layer 124a in the first surface 110, layer 124b in the second surface 212 and layers 124c and 124d in the opposing side surfaces 122. For clarity of illustration, the inner edges of the non-cellular surface layers 124 are shown by dashed lines in Figure 6.

[056] In Figure 6, the fusion-bonded connection 130 is illustrated highly schematically by a series of solid elements, and the width dimensions are not to scale and are exaggerated for the purpose of clarity of illustration. For clarity of illustration, the inner edges of the non-cellular surface layers 124, which are at the boundary between the non-cellular surface layers 124 and the expanded cellular foam 108, are shown by dashed lines in Figure 6.

[057] The fusion-bonded connection 130 comprises a plurality of individual, and mutually spaced, bond areas 132 distributed, preferably randomly, across the dividing region 116.

[058] Typically, the fusion-bonded connection 130 has a tensile strength of from 0.05 to 4.0 MPa, typically from 0.07 to 0.12 MPa.

[059] In use the core material 104 is provided in a planar configuration, similar to Figure 3, and the fusion-bonded connections 130 between the segments maintain the planar configuration of the core material 104 during normal shipping and handling. When the core material 104 is being laid up into a mould for the manufacture of a sandwich panel, for example for the manufacture of a wind turbine blade, in regions where the core material 104 requires a curved shape to fit a curvature of the mould surface, then individual fusion-bonded connections 130 can be selectively broken or “snapped”, to form a curved configuration as shown in Figure 7, since the fusion-bonded connection 130 is breakable by flexing the adjacent segments 114 about the respective flexible hinge 128.

[060] The provision of the weakly bonded fusion-bonded connections 130 enables some parts of the core material 104 which require curvature to be selectively flexed by “snapping” the weak fusion bonds between adjacent segments 114, whereas other parts of the core material 104 which are to remain planar can retain the fusion-bonded connections 130 between adjacent segments 114, and thereby retain structural integrity in the planar parts.

[061] By retaining the fusion-bonded connections 130 between adjacent segments 114 in the planar parts of the resultant sandwich panel, any liquid resin infusion into the core material 104 at the planar parts is reduced or eliminated, because the adjacent segments 114 remain fused together and therefore fewer internal grooves or gaps are formed between the adjacent segments 114, into which liquid resin could be received.

[062] Therefore the provision of dividing regions 116 between adjacent segments 114 which can be “snapped” open or remain closed can reduce liquid resin infusion into the core material 104 during the manufacture of a sandwich panel.

[063] Referring to Figure 8, there is schematically illustrated a section of a core material 204 for a sandwich panel in accordance with a third embodiment of the present invention. The core material is shown in an unloaded state, in a planar configuration. For clarity, again some dimensions are not to scale and are exaggerated for clarity of illustration.

[064] In this embodiment, the core material 204 has substantially the same general composition and structure as the core material 104 of the second embodiment, except that in the third embodiment an external hinge is provided instead of the integral flexible hinge formed by the non-cellular surface layer at the first surface of the core layer.

[065] In Figure 8, the fusion-bonded connection 230 is again illustrated highly schematically by a series of solid elements, and the width dimensions are not to scale and are exaggerated for the purpose of clarity of illustration. For clarity of illustration, the inner edges of the non-cellular surface layers 224, which are at the boundary between the non-cellular surface layers 224 and the expanded cellular foam 208, are again shown by dashed lines in Figure 8.

[066] The fusion-bonded connection 230 comprises a plurality of individual, and mutually spaced, bond areas 232 distributed, preferably randomly, across the dividing region 216. Typically, the fusion-bonded connection 230 has the same tensile strength as described above for the first embodiment.

[067] The non-cellular surface layers 224, as in the previous embodiments, are formed as layer 224a in the first surface 210, layer 224b in the second surface 212 and layers 224c and 224d in the opposing side surfaces 222. The non-cellular surface layer 224a at the first surface 210 of the core layer 206 does not form an integral hinge element. Instead, a flexible outer layer 240 is attached to the first surface 210. The outer layer typically comprises a fabric, for example a scrim material. Typically a polyester scrim material, for example an unwoven PET scrim having low areal weight, is attached, e.g. by an adhesive, to the first surface 210. The flexible outer layer 240 functions to provide hinged connections, by flexible hinges 242, between the adjacent segments 214.

[068] As shown in Figure 9, which for clarity does not show the non-cellular surface layers 224 shown in Figure 8, when the core layer 204 is curved about an axis transverse to the longitudinal direction by flexing the core layer 204 about one or more flexible hinges 242 formed by the outer layer 240 at the respective dividing regions 216, the adjacent segments 214 are hingedly separated along respective dividing regions 216 by breaking the respective fusion-bonded connection 230, leaving residual snapped portions 234 of the bond areas 232.

[069] Various methods for manufacturing a core material for a sandwich panel in accordance with embodiments of the present invention, in particular the first to second embodiments disclosed with reference to Figures 1 to 9, will now be described.

[070] Referring to Figure 10, there is schematically shown a process flow for manufacturing core materials in accordance with an embodiment of the method of the present invention.

[071] An elongate body 50 of extruded expanded cellular foam, composed of PET, is outputted from an extruder 52, which is generally known to those skilled in the art. The body 50 is formed as a layer 54 with upper and lower planar surfaces 56, 58. The layer 54 is fed through a calibrator 60 which determines the width and thickness dimensions of the layer 54.

[072] Thereafter, the elongate layer 54 is fed to a slicing apparatus 62 at which the elongate layer 54 is cut into a plurality of individual planks 64 of desired length. The planks 64 are then cooled to ambient temperature in a cooling tower (not shown). The cooled planks 64 are then planed to a desired thickness by a planing machine 66, to form expanded cellular foam panels 68 having the desired length, width and thickness. The panels 68 are typically stacked for further processing and / or transportation / supply prior to subsequent processing. All of these steps are well known to those skilled in the art.

[073] A plurality of the panels 68 is assembled to form a stack 70 of the panels 68. The panels 68 in the stack 70 may be aligned and trimmed as required to provide a trimmed stack 72 in which the panels 68 have identical length and width dimensions as a result of the trimming operation, and the same thickness as a result of the planing operation.

[074] As described in the next paragraph, the plurality of trimmed panels 68 is then processed to seal the opposite faces of the panels 68 to provide, by heating, a non-cellular surface layer of the thermoplastic polymer on each of the opposite faces of the panels 68. The non-cellular surface layer forms a sealing layer on the surface of the expanded cellular foam, which is provided on the opposed side surfaces 22 of the dividing regions 16 of the core materials as described above for the first to third embodiments.

[075] Accordingly, the method provides a plurality of a panels 68 of the expanded cellular foam, each panel 68 having opposite planar surfaces 74, 76. A pair of the panels 68, are disposed, as first and second panels 68a, 68b, on opposite sides of a heating device 78 so that each of the first and second panels 68a, 68b has one of the planar surfaces 74, 76 thereof facing a respective heated surface 80, 82 of the heating device 78. Heat is applied from the heating device 78 to the facing surfaces 74, 76 of the first and second panels 68a, 68b to heat the thermoplastic polymer and deforming the heated thermoplastic polymer to form a non-cellular surface layer, as described above, but not expressly shown in Figure 10, of the thermoplastic polymer. The formation of the non-cellular surface layer seals the respective surface of the cellular foam.

[076] The heating device may apply heat by a variety of different mechanisms, for example by direct contact and thermal conduction from a heated element, or by thermal radiation, e.g. infra-red heating or by radiation from a heated body. In this embodiment, the facing surfaces 74, 76 of the first and second panels 68a, 68b are directly contacted by a respective heated surface 80, 82 of the heating device 78, which is a heating member in the form of a heated metal plate having planar heated surfaces.

[077] In order to form the non-cellular surface layer, which then functions as a sealing layer which at least partly closes the open pores of the cellular foam, the thermoplastic polymer of the cellular foam is preferably heated to a temperature which is equal to or above the melting temperature of the thermoplastic polymer and then contacted to enable the thermoplastic polymer to be inelastically deformed so as at least partly to close the open pores of the cellular foam.

[078] Preferably, each heated surface 80, 82 of the heating device 78 is at temperature which is within a range which extends from a lower value, which is the melting temperature of the thermoplastic polymer, to an upper value, which is up to 30 °C above the melting temperature of the specific thermoplastic polymer forming the cellular foam. As is known to those skilled in the art of thermoplastic polymers, different grades of the same polymer may have different melting points; for example, commercially available polyethylene terephthalate has different grades which may exhibit different melting points and therefore effectively PET polymers exhibit a range of melting points. When the thermoplastic polymer comprises polyethylene terephthalate, which, depending on the grade of PET typically has a melting temperature of from 230 to 260 °C, typically the heated surface 80, 82 is at temperature which is within the range of from 230 to 280 °C, optionally from 230 to 260 °C.

[079] Typically, the facing surfaces 74, 76 of the first and second panels 68a, 68b are directly contacted by a respective heated surface 80, 82 of the heating device 78 for a period of from 5 to 40 seconds, for example from 15 to 20 seconds. The facing surfaces 74, 76 of the first and second panels 68a, 68b are preferably directly contacted by a respective heated surface 80, 82 of the heating device 78 at an applied contact pressure of from 0.001 to 0.85 MPa. For example, a pressure of from 0.001 to 0.01 MPa is applied against the at the facing surfaces 74, 76 of the first and second panels 68a, 68b.

[080] After the heating step, preferably the heated facing surfaces 74, 76 of the first and second panels 68a, 68b are subjected to a dwell step in which the heated facing surfaces 74, 76 are removed from exposure to the heated surfaces 84 of the heating device 78 and permitted to cool prior to a subsequent step of pressing together the heated facing surfaces 74, 76. In some embodiments, the heating step can be controlled so that the heated facing surfaces 74, 76 of the first and second panels 68a, 68b can be heated to a specific desired heated condition so that the dwell can be omitted prior to the pressing step. [081 ] Then the non-cellular surface layers on the facing surfaces 74, 76 of the first and second panels 68a, 68b are pressed together, shown by arrows P, to form a block 86 of the panels 68.

[082] The panels 68 are individually taken from the stack 70 and then a first pair of panels 68 are heated, and then assembled together to form the block 86. Then a next panel 68 is individually taken from the stack 70 and a forward facing surface 76 of that panel 68 and a rearwardly facing surface 74 of the last panel assembled into the block 86 are simultaneously heated, and then pressed together to be assembled into the block 86 and increase the number of panels in the block 86. In this way, the block 86 is successively built up, panel by panel, with opposing layers of the panels 68 being sealed by heat-sealing.

[083] The heating, optional dwell, and pressing steps are repeated on additional panels 68 of the plurality of panels 68 to successively form the block 86 of the panels 68, as shown in Figure 10. The cutting element may comprise a blade or any other cutting element known for use in cutting expanded foam. In the block 86, adjacent panels 68 of the block 86 comprise opposed non-cellular surface layers of the thermoplastic polymer, as described above.

[084] After the block 68 is formed with the desired number of panels 68, a cutting element 88 is employed to cut through the block 86 of panels 68 along a cutting line 90 to form a plurality of layers 92. Each layer 92 comprises a series of the segments 314, wherein each segment 314 is formed from a cut portion 94 of one of the respective panels 68.

[085] In accordance with the method of the present invention, the parameters of the heating and dwell steps may be varied depending on the structure and properties required for the final core material.

[086] For example, when manufacturing the core material of the first embodiment as described above with reference to Figures 1 to 5, in which the opposing side surfaces 22 of the adjacent segments 14 which are in mutual contact are free of any bonding therebetween, the dwell step is controlled to have a sufficient time period so that before the pressing step the facing surfaces 74, 76 of the first and second panels 68a, 68b have sufficiently cooled so that when the facing surfaces 74, 76 are pressed together no fusion bond is formed between the opposing side surfaces 22.

[087] Therefore, in this embodiment of the method of the invention, the dwell step is carried out for a period sufficient to cool the heated facing surfaces 74, 76 to solidify the non-cellular surface layers 24 of the thermoplastic polymer prior to the pressing step, whereby after pressing step and subsequent cooling to ambient temperature, the opposed facing surfaces 74, 76 of the adjacent segments 314 which are in contact are free of any bonding therebetween. Typically, the dwell step is carried out for a period of greater than 15 seconds, typically from greater than 15 to up to 30 seconds, to avoid forming any fusion bond.

[088] In the embodiment in which the opposed facing surfaces 74, 76 of the adjacent segments 314 which are in contact are free of any bonding therebetween, as the block 86 is formed the adjacent panels 68 are held together by a frame (not shown). The frame holds together the panels 68 during the cutting step and in the resultant layer 92.

[089] Alternatively, when manufacturing the core material of the second and third embodiments as described above with reference to Figures 6 and 7 and Figures 8 and 9, in which the opposing side surfaces 22 of the adjacent segments 14 which are in mutual contact are weakly bonded by a fusion-bonded connection therebetween, the dwell step is controlled to have a time period so that when the pressing step is carried out the facing surfaces 74, 76 of the first and second panels 68a, 68b have maintained sufficient thermal energy so that the polymer surfaces are still deformable and fusible whereby when the facing surfaces 74, 76 are pressed together, the desired fusion-bonded connection is formed between the opposing side surfaces 22.

[090] Therefore, in these embodiments of the method of the invention, the dwell step is typically carried out for a period of from 5 to 15 seconds, optionally from 5 to 10 seconds, and during the subsequent pressing step the heated facing surfaces 74, 76 are bonded together by a fusion-bonded connection therebetween. The provision of this combination of heating and dwell steps provides that after the pressing step, and subsequent cooling to ambient temperature, the opposed facing surfaces 74, 76 are weakly bonded together by the fusion-bonded connection therebetween. As described above, in the final core materials produced in accordance with this embodiment, the fusion-bonded connection is broken by flexing the adjacent segments about the respective flexible hinge.

[091] As described above for the core material of the second and third embodiments, the fusion-bonded connection typically comprises a plurality of individual, and mutually spaced, bond areas distributed, typically randomly, across the facing surfaces 74, 76.

[092] The random distribution of the bond areas may be achieved simply as a result of the phenomenon that the heating surfaces 80, 82 of the heating device 78 may randomly heat areas of the facing surfaces 74, 76 to slightly different temperatures, and / or that the heated areas may randomly cool at different rates than prior to the pressing step. These effects may result from the fact that a cellular body, with voids and cell walls, is disposed immediately behind the thin non-cellular surface layer, and the inevitable minor variations in the cellular structure may result in minor variations in the heating and cooling of the surface layers prior to pressing.

[093] Alternatively, the heating surfaces 80, 82 of the heating device 78 may be provided with non-uniform structure and / or properties so that the bond areas may be provided under controlled conditions, for example either as a regular or random array. For example, the heating surfaces 80, 82 may be provided with depressed regions (not shown) which may locally cause reduced heating of the facing surfaces 74, 76 as compared to areas of the heating surfaces 80, 82 surrounding the depressed regions.

[094] As described above, each layer 92 comprises a series of the segments 314, wherein each segment 314 is formed from a cut portion 94 of one of the respective panels 68. In particular, the plurality of segments 314 of expanded cellular foam composed of a thermoplastic polymer are assembled to form the layer 92. The adjacent segments 314 are in mutual contact by respective opposite planar side surfaces 322 which extend through the thickness of the layer 92. The thickness extends between opposite first and second surfaces 310, 312 of the layer 92. The side surfaces 322 are transverse to a longitudinal direction of the layer 92 which extends between opposite first and second ends 318, 320 of the layer 92. The planar side surfaces 322 of the adjacent segments 314 which are in contact each comprise a non-cellular surface layer of the thermoplastic polymer, for example non-cellular surface layers 24c and 24d as described above in the first to third embodiments.

[095] In a subsequent step in the method of the invention, heat is applied to the first surface 310 of the panel 92 and the heated thermoplastic polymer is deformed to form a non-cellular surface layer of the thermoplastic polymer which extends inwardly of the first surface. The non-cellular surface layer may optionally form a flexible hinge between adjacent segments as described above for the first and second embodiments of the core material of the invention, whereby the segments are interconnected at the first surface by a series of the flexible hinges to form a core layer which may be curved about an axis transverse to the longitudinal direction by flexing the core layer about one or more flexible hinges thereby hingedly to separate adjacent segments. Alternatively, the non-cellular surface layer does not form a flexible hinge between adjacent segments, but instead the flexible hinge is provided by attachment of a flexible outer layer to the first surface as described above for the third embodiment of the core material of the invention.

[096] In this heating step, typically the first surface 310 of the layer 92 is directly contacted by a heated surface 350 of a heating member 352. In the illustrated embodiment, the heating member 352 is a roller and the heated surface 350 is cylindrical. In an alternative embodiment, the heating member 352 may be a plate and the heated surface 350 is planar. The heated surface 350 is at temperature which is within a range which extends from a lower value, which the melting temperature of the thermoplastic polymer, to an upper value, which is up to 30 °C above the melting temperature of the thermoplastic polymer.

[097] Typically, when the thermoplastic polymer comprises polyethylene terephthalate, the heated surface 350 is at temperature which is within the range of from 230 to 280 °C, preferably from 230 to 260 °C. In the illustrated embodiment, the first surface 310 of the layer 92 is directly contacted by the heated surface 350 of the heating member 352 for a period of from 0.1 to 20 seconds, for example from 0.3 to 0.6 seconds.

[098] When the first surface 310 of the layer 92 is directly contacted by the heated surface 350 of the heating member 352, preferably the applied contact pressure is from 0.001 to 72 MPa, for example from 0.001 to 0.11 MPa. In alternative embodiment, the first surface 310 may be heated indirectly by radiant heating, e g. by infra-red radiation.

[099] As described with reference to the first and second embodiments, the heating may be controlled to form a non-cellular surface layer of the thermoplastic polymer at the first surface which, after cooling to ambient temperature, forms a flexible hinge between adjacent segments 314. As described above, the flexible hinge typically has a thickness extending inwardly from the first surface 310 of from 0.1 to 3 mm, for example from 0.25 to 0.5 mm. The flexible hinge preferably consists of the non-cellular surface layer formed at the first surface 310.

[100] In the heating step applied to layer 92, preferably simultaneously the second surface 312 is heated and the thermoplastic polymer is deformed to form a non-cellular surface layer of the thermoplastic polymer which extends inwardly of the second surface 312. As for the heating of the first surface 310, typically the second surface 312 of the layer 92 is directly contacted by a heated surface 354 of a second heating member 356, which may be a roller with a cylindrical heated surface 354, as illustrated, or alternatively a plate with a planar heated surface. Again, radiant heating, e.g. infra-red radiation, may alternatively be used.

[101] Typically, the heated surface 354 of the second heating member 356 is at temperature which is within a range which extends from a lower value, which is the melting temperature of the thermoplastic polymer, to an upper value, which is up to 30 °C above the melting temperature of the thermoplastic polymer. When the thermoplastic polymer comprises polyethylene terephthalate, typically the heated surface 354 of the second member 356 is at temperature which is within the range of from 230 to 280 °C.

[102] Typically, the second surface 312 of the layer 92 is directly contacted by the heated surface 354 of the second heating member 356 for a period of from 10 to 30 seconds, for example from 0.1 to 20 seconds. As for the first surface 310, when the second surface 312 of the layer 92 is directly contacted by the heated surface 354 of the second heating member 356, typically the applied contact pressure is from 0.001 to 0.85 MPa, for example from 0.001 to 0.01 MPa.

[103] Figure 11 schematically illustrates an alternative apparatus for sealing the surface of cellular foam to form a core layer. The core layer may have the structure shown for the core layer 506 in Figure 12. The apparatus of Figure 11 comprises a heating arrangement which can be used as, or alternatively to both (i) the heating device 78 illustrated in Figure 10 and (ii) the first and second heating members 352, 356, each in the form of a roller, also illustrated in Figure 10.

[104] In the apparatus of Figure 11, designated generally as 400, a support table 402 is provided which has an upper surface 404 for slidingly supporting a cellular foam layer 492, by contacting the lower planar surface 496 of the cellular foam layer 492.

[105] As described below with reference to Figures 12 and 13, the cellular foam layer 492 may comprise (i) a cellular foam panel, corresponding to the cellular foam panel 68 illustrated in Figure 10, or (ii) a layer, corresponding to the cellular foam layer 92 illustrated in Figure 10, which comprises a series of segments assembled together to form the layer.

[106] A drive system 406 is located above the support table 402, and is spaced above the upper surface 404 so that the drive system 406 engages the upper planar surface 498 of the cellular foam layer 492. In the illustrated embodiment, the drive system 406 comprises an endless belt 408 mounted on a pair of spaced drive rollers 410. The drive system 406 is configured to translationally drive the cellular foam layer 492 in a horizontal direction as shown by the arrow H.

[107] An infra-red lamp 412 is positioned downstream, in the direction H, of the support table 402 and the drive system 406. The infra-red lamp 412 extends across the transverse width of the cellular foam layer 492, and is configured to irradiate the entire width of the lower planar surface 496 of the cellular foam layer 492 as the cellular foam layer 492 is translationally moved over the infra-red lamp 412 in the direction H. The infra-red lamp 412 heats the lower planar surface 496 of the cellular foam layer 492 to above the melting temperature Tm of the thermoplastic polymer, and causes the thermoplastic polymer at the surface 496 to melt.

[108] The infra-red lamp 412 irradiates the lower planar surface 496 of the cellular foam layer 492 with thermal energy in order to cause a surface layer 500 of a desired thickness to melt. The selected thermal energy density is dependent upon the thickness of the molten surface layer 500 to be formed, the density of the cellular foam layer 492, and the translational speed of the cellular foam layer 492 which is driven by the drive system 406. These parameters can readily be determined experimentally by the skilled person for any given cellular foam to achieve a given thickness of the molten surface layer.

[109] In the final core layer 506, the sealed non-cellular surface layer has a thickness, which is typically from 0.05 to 1.5 mm, and preferably from 0.1 to 0.3 mm. In order to achieve this desired thickness in the final core layer 506, typically the infra-red lamp 412 irradiates the lower planar surface 496 of the cellular foam layer 492 within a thermal energy density range of from 37.5 to 150 kW / m2 when the translational speed of the cellular foam layer 492 is within a range of from 0.3 to 3 m / min; preferably, the thermal energy density range is from 75 to 100 kW / m2 and the translational speed of the cellular foam layer 492 is within a range of from 1 to 1.5 m / min.

[110] A scraper element 414, having an uppermost scraper surface 416, is positioned downstream, in the direction H, of the infra-red lamp 412. The scraper surface 416 extends across the transverse width of the cellular foam layer 492, and is configured engage the heated thermoplastic polymer to deform the molten thermoplastic polymer at the surface 496 and form a continuous surface film of molten thermoplastic polymer at the lower planar surface 496 of the cellular foam layer 492 as the cellular foam layer 492 is translationally moved over the scraper element 414, in the direction H. The scraper surface 416 directly contacts the lower planar surface 496 of the cellular foam layer 492. [Ill] As shown in Figure 11, the scraper surface 416 is located at a distance X downstream, in the direction H, from the infra-red lamp 412; in particular the centre, in direction H, of the scraper surface 416 is located at distance X downstream from the centre, in direction H, of the infra-red lamp 412. Typically, distance X is within the range of from 20 to 100 mm, for example from 25 to 35 mm, most typically about 30 mm.

[112] A roller 418 is located above the cellular foam layer 492 downstream of the scraper element 414. The roller 418 is undriven, and extends across the width of the cellular foam layer 492. The roller 418 applies a downward force onto the upper planar surface 498 of the cellular foam layer 492, which thereby controls the upward force applied by the scraper surface 416 to the lower planar surface 496 of the cellular foam layer 492. Typically, the scraper surface 416 of the scraper element 414 is applied against the lower planar surface 496 of the cellular foam layer 492 at an applied contact pressure of from 0.001 to 6.7 MPa.

[113] As shown in Figure 11, the roller 418 is aligned with, or located at a distance Y downstream, in the direction H from, the scraper surface 416; in particular the centre, in direction H, of the roller 418 is located at distance Y downstream from the centre, in direction H, of the scraper surface 416. Typically, distance Y is within the range of from 0 to 300 mm, for example from 175 to 225 mm, most typically about 200 mm; when the distance Y is 0 mm, the roller 418 is aligned with, and in the illustrated embodiment, located directly above, the scraper surface 416, which would also prevent any bending of the cellular foam layer 492 as a result of application of the upwardly-oriented scraping force.

[114] The surface layer 500 of molten thermoplastic polymer at the lower surface 498 solidifies to form a sealed non-cellular surface layer, as described above.

[115] The roller 418 is configured to apply a desired line force against the upper planar surface 498 of the cellular foam layer 492 in order for the scraper element 414 to form a continuous surface layer 500 of molten thermoplastic polymer at the lower planar surface 496 of the cellular foam layer 492 of the desired thickness, which is typically from 0.05 to 1.5 mm, and preferably from 0.1 to 0.3 mm. The selected line force applied by the roller 418 is dependent upon the thickness of the cellular foam layer 492, and the distance Y. The line force is expressed as the force applied per unit width of the cellular foam layer 492, the roller 418 extending parallel to the width direction. Typically, the line force is within the range of from 10 to 80 N / m, for example 50 to 70 N / m, most typically about 60 N / m; such a line force may be applied when, for example, the distance Y is a value of 200mm.

[116] A shape controller 420 is located above the roller 418 downstream of the roller 418. The shape controller 420 comprises a planar element which extends across the width of the cellular foam layer 492. The shape controller 420 applies a further downward force onto the upper planar surface 498 of the cellular foam layer 492, which thereby keeps the cellular foam layer 492 in a flat configuration so that the cellular foam layer 492 exiting the apparatus 400 is fully solidified and planar, and is cooled to approximately room temperature so that no undesired thermal distortion is caused after the cellular foam layer 492 has exited the apparatus 400. The support table 402, or an extension thereof or a further support table 402a as shown in Figure 11, may extend beneath the cellular foam layer 492 at a location under the shape controller 420 so that the cellular foam layer 492 is supported by the support table 402, extension or further support table 402a as the downward force is applied by the shape controller 420.

[117] As explained above, the apparatus 400 may be used to treat a cellular foam surface of (i) a cellular foam panel, corresponding to the cellular foam panel 68 illustrated in Figure 10, or (ii) a layer, corresponding to the cellular foam layer 92 illustrated in Figure 10, which comprises a series of segments assembled together to form the layer. In each case, the apparatus 400 forms a sealed non-cellular surface layer.

[118] Referring to Figures 12 and 13, the apparatus may be used to produce the core layer of Figure 12 using the process flow of Figure 13. For clarity of illustration, Figure 12 only shows two adjacent segments 514a, 514b, although the final core layer 506 comprises a larger number of serially arranged segments, as shown in Figure 10. Also, for clarity of illustration, dimensions are not to scale in Figure 12.

[119] Initially, foam panels are provided, corresponding to the foam panels 68 produced in the apparatus of Figure 10. Each foam panel is conveyed through the apparatus 400 to form a respective sealed non-cellular surface layer. In Step 1, the foam panel, functioning as the cellular foam layer 492 shown in Figure 11, is conveyed through the apparatus 400 to produce the sealed non-cellular surface layer 530 on one side of the foam panel; the sealed non-cellular surface layer 530 is to form the side surface of a segment 514a, 514b, etc in the final core layer 506, as shown in Figure 12. In Step 2, subsequently the foam panel is conveyed again through the apparatus 400 to produce the sealed non-cellular surface layer 532 on the opposite side of the foam panel; the sealed non-cellular surface layer 532 is to form the opposite side surface of a segment 514a, 514b, etc in the final core layer 506, as also shown in Figure 12.

[120] Steps 1 and 2 are carried out sequentially, as shown in the process flow of Figure 13. When the apparatus 400 of Figure 11 is used, the foam panel is passed through the apparatus 400 twice, a first pass forming the sealed non-cellular surface layer 530 on one side of the foam panel and the second pass forming the sealed non-cellular surface layer 532 on the opposite side of the foam panel. However, the apparatus 400 may be modified to provide two longitudinally spaced heating / scraping stations, each of which is configured to treat a respective surface of the foam panel. Consequently, with this alternative modified apparatus, only a single translational pass is required to carry out Steps 1 and 2 so that both opposed surfaces of the foam panel are provided with a respective sealed non-cellular surface layer in a single pass.

[121] After the opposite surfaces of the foam panel have been provided with a respective sealed non-cellular surface layer 530, 532, the planks are assembled into a block, as shown by block 86 in Figure 10.

[122] The assembly and cutting steps to form a foam layer comprising adjacent segments, shown by Steps 3 and 4 in Figure 13, may be carried out according to various alternative embodiments.

[123] In a first alternative embodiment, a cutting step 3a is followed by an assembling step 4a. In a second alternative embodiment, an assembling step 3b is followed by a cutting step 4b.

[124] In the first alternative embodiment, in Step 3a the sealed panel is cut into strips, each strip having opposite upper and lower sealed surfaces provided by the sealed non-cellular surface layers 530, 532. Then in Step 4a, the strips are each rotated by an angle of 90 °, and the rotated strips are assembled together to form an assembled layer, in which the sealed non-cellular surface layer 530 of a given strip is in contact with the sealed non-cellular surface layer 532 of the adjacent strip. Consequently, in the assembled layer of strips, each strip provides a respective segment 514a, 514b, etc, as described above for foam layer 92 illustrated in Figure 10, and the adjacent pairs of sealed non-cellular surface layers 530, 532 extend through the thickness of the assembled layer. In the assembly Step 4a, an external frame (not shown) may be provided to assist assembly of the series of adjacent strips.

[125] In the second alternative embodiments, in Step 3b the sealed panels produced in Step 2 are assembled together to form a block. An external frame (not shown) may be provided to hold the foam panels together as a block. Thereafter, in Step 4b, the block, composed of the assembled foam panels, is transversely cut, as described above with reference to Figure 10, to form a plurality of foam layers, each foam layer comprising a plurality of segments 514a, 514b, etc, as described above for foam layer 92 illustrated in Figure 10.

[126] In each of these first and second alternative embodiments, in the final core layer 506 the opposed sealed non-cellular surface layers 530, 532 at the dividing region 516 are not bonded together. In the assembly Step 3a or the assembly Step 4b, the individual foam strips or panels may be assembled sequentially after allowing the sealed non-cellular surface layers 530, 532 to cool, so that when the opposed sealed non-cellular surface layers 530, 532 are urged together, for example to add a further strip to the layer in Step 3a or to add a further foam panel to the block in Step 4b, the non-cellular surface layer 530 of the added foam strip or panel is not fusion-bonded to the facing non-cellular surface layer 532 of the layer or block. Instead, the strips or panels are held together by a frame |(not shown).

[127] In subsequent Step 5 following either of Steps 4a or 4b, the foam layer, comprising the series of segments 514a, 514b, etc, is conveyed through the apparatus 400 to form a sealed non-cellular surface layer 534 and hinge 528, as described above with reference to Figure 12, on one side of the foam layer. In Step 6, subsequently the foam layer is conveyed again through the apparatus 400 to produce the sealed non-cellular surface layer 536 on the opposite side of the foam layer, as also shown in Figure 12.

[128] Steps 5 and 6 are carried out sequentially, as shown in the process flow of Figure 13. When the apparatus 400 of Figure 11 is used, the foam layer is passed through the apparatus 400 twice, a first pass forming the sealed non-cellular surface layer 534 and hinge 528 on one side of the foam layer and the second pass forming the sealed non-cellular surface layer 536 on the opposite side of the foam layer. However, the apparatus 400 may be modified to provide two longitudinally spaced heating / scraping stations, each of which is configured to treat a respective surface of the foam layer. Consequently, with this alternative modified apparatus, only a single translational pass is required to carry out Steps 5 and 6 so that both opposed surfaces of the foam layer are provided with a respective sealed non-cellular surface layer in a single pass. Steps 5 and 6 as described above produce the foam core 506 shown in Figure 12.

[129] In a further alternative embodiment, Step 6 may be omitted if the foam core is intended to be sealed, by a respective non-cellular surface layer 534, on surface 510 incorporating the flexible hinge 528 but is not sealed on opposite surface 512.

[130] In a further alternative embodiment, in Step 5 a scrim material layer, corresponding to the scrim material forming the flexible outer layer 240 shown in Figure 9, may be applied to one surface 510 of the foam layer instead of forming the non-cellular surface layer 534 on surface 510. The scrim material layer functions to provide a series of flexible hinges along the surface 510. The opposite surface 512 may be sealed as described in Step 6, or not sealed as described in the preceding paragraph.

[131] The present invention is now further described with reference to the following Examples. Examples 1 and 2, and Comparative Example 1

[132] A kit of PET foam parts for the manufacture of a typical wind turbine blade having a length of 98 m was modelled using computer modelling known to those skilled in the art of manufacturing wind turbine blades. The kit of PET foam parts is used to make a sandwich panel structure on the opposite blade surfaces.

[133] In Comparative Example 1, the kit comprised PET foam layers having cuts sawn into the thickness of the PET foam layer to provide integral foam hinges which in turn provided the desired flexibility to the kitted parts. The modelled cuts were determined as having a groove width of 1.2 mm.

[134] The modelling was used to calculate the resin weight savings contribution from potential sealing of the sealed surfaces of the unwelded dividing regions between adjacent segments; any potential weight saving as a result of sealing the opposite major surfaces of the core layer which are bonded to the opposite composite material plies was excluded from the estimated savings in the modelled system, because these surfaces are typically sealed in a conventional core layer normal sealing process.

[135] After modelling of a conventional blade manufacturing process using resin transfer moulding (RTM), the total resin uptake by the blade was calculated as 2028.1 kg, of which 1056.6 kg was attributed to the resin uptake through the surface area of the cellular foam and 971.5 kg was attributed to the resin uptake as a free volume external of the cellular foam. These results are shown in Table 1.

[136] In Example 1, the kit comprised PET foam layers having sealed surfaces and unwelded dividing regions between adjacent segments, as described above for the first embodiment of Figures 1 to 5. The modelled contacting surfaces at the dividing regions were determined as having a nominal potential width of 0.3 mm. The PET foam layer provided integral foam hinges in the kitted parts. After a repeat modelling of the same blade manufacturing process using resin transfer moulding (RTM) as used in Comparative Example 1, the total resin uptake by the blade was calculated as 1193.6 kg, of which 735.5 kg was attributed to the resin uptake through the surface area of the cellular foam and 458.0 kg was attributed to the resin uptake as a free volume external of the cellular foam. These results are also shown in Table 1.

[137] Table 1 Comparative Example 1 Example 1 Example 2 Resin take up surface area 1056.6 735.5 646.1 Resin take up - free volume 971.5 458.0 410.4 Total resin take up 2028.1 1193.6 1056.6

[138] A comparison of Example 1 and Comparative Example I shows that the present invention can provide a core layer which can, for a typical wind turbine blade, achieve a reduction in total resin take up of about 834.6 kg. The significantly reduced resin uptake through the surface area of the cellular foam is achieved because the transverse surfaces extending partially but substantially through the foam layer are sealed, rather than being cut surfaces which expose open cells, and so a significantly less liquid resin is absorbed into the cellular foam through these surfaces. The significantly reduced resin uptake as a free volume external of the cellular foam is achieved because the transverse surfaces extending partially but substantially through the foam layer are in contact, rather than separated as is provide by a cut groove where the cutting process removes material to leave a gap between the opposed cut surfaces. In the present invention, the sealed surfaces are pressed together to be in contact, and so any gap therebetween is eliminated or minimized, thereby reducing the free volume for receiving liquid resin during the resin infusion process. This total reduced resin take up corresponds to an average reduction in resin take up about 1.4 kg / m2 of the blade surface. Therefore, the core material of the present invention can achieve a significant reduction in resin take up, and a significant reduction in the total resin weight incorporated into the wind turbine blade, as compared to a known foam kit, without any reduction in flexibility of the foam parts in the kit.

[139] In Example 2, the kit comprised PET foam layers having sealed surfaces and weakly welded dividing regions between adjacent segments, as described above for the second embodiment of Figures 6 and 7. The PET foam layer provided integral foam hinges in the kitted parts. After a repeat modelling of the same blade manufacturing process using resin transfer moulding (RTM) as used in Comparative Example 1 and Example 1, the total resin uptake by the blade was calculated as 1056.6 kg, of which 646.1 kg was attributed to the resin uptake through the surface area of the cellular foam and 410.4 kg was attributed to the resin uptake as a free volume external of the cellular foam. These results are also shown in Table 1.

[140] A comparison of Example 2 and Example 1 shows that the additional provision of the weakly welded dividing regions to provide manually breakable “snappable ” fusion-bonded connections between adjacent segments further reduces resin take up without reducing flexibility of the core panel, and providing the additional advantage that in desired planar regions the core panel can be retained in a coherent, continuous planar state. The further reduced resin uptake through the surface area of the cellular foam is achieved because the retention of some unsnapped dividing regions in the moulded product still further reduces the liquid resin absorbed into the cellular foam through these surfaces. The further reduced resin uptake as a free volume external of the cellular foam is achieved because the retention of some unsnapped dividing regions in the moulded product still further reduces the total volume of any gap between the sealed surfaces. Therefore, the core material of this embodiment of the present invention can achieve a further reduction in resin take up, and a further reduction in the total resin weight incorporated into the wind turbine blade, as compared to a known foam kit, without any reduction in flexibility of the foam parts in the kit. Examples 3 6 and Comparative Examples 2-5

[141] A kit of PET foam parts for the manufacture of a typical wind turbine blade having a curvature of different radius values was modelled using computer modelling known to those skilled in the art of manufacturing wind turbine blades. The kit of PET foam parts is used to make a sandwich panel structure on the opposite blade surfaces. In Examples 3 to 6, the core layer had the structure disclosed in Figures 1 to 5 with the integral hinges mutually spaced by 50mm. The nominal width of any spacing between the opposed sealed surfaces in contact at the dividing regions, incorporating the integral hinge, was determined as 0.3 mm, and when the core layer is curved about a radius this width is enlarged by the formation of a gap as a result of the curvature. The size of the gap increases with decreasing radius of curvature. In Comparative Examples 2 to 5, the kit of PET foam parts had conventional grooves cut into the core layer, with a width determined as 1.2 mm, a depth corresponding to the core thickness minus 2mm to provide an uncut foam portion, and a groove spacing of 50mm for direct comparison.

[142] In Example 3, the core layer was flexed about a radius of curvature of 10 m, and the resin uptake in the resultant “grooves”, i.e. the gaps formed by the curved morphology, was determined for various core thickness values ranging from 5 to 80 mm. In Comparative Example 2 the core layer was also flexed about a radius of curvature of 10 m, and the resin uptake in the cut grooves, which were enlarged by the curved morphology, was also determined for various core thickness values ranging from 5 to 80 mm. Figure 14 (a) shows these results, and shows that the use of the contacting sealed surfaces at the series of dividing regions significantly reduces resin uptake in the curved core layer as compared to a conventional kitting solution using cut grooves. The absolute weight saving as a result of reduced resin uptake increases with increasing core thickness, because the cut grooves are increased in length with greater core thickness. In Figure 14(a), the term “UwldlOm” refers to an “unwelded groove” having opposed sealed surfaces above an integral hinge in accordance with the embodiment of Figures 1-5 and the core layer being flexed about a radius of curvature of 10 m; and the term “DGlOm” refers to a “deep groove” having opposed cut surfaces as described above for Comparative Examples 2-5 and the core layer being flexed about a radius of curvature of 10 m.

[143] In Figure 14 (b), Example 4 and Comparative Example 3 show similar results when the core layer of varying thickness is flexed about a radius of 5 m; in Figure 14 (c), Example 5 and Comparative Example 4 show similar results when the core layer of varying thickness is flexed about a radius of 2m; and in Figure 14 (d), Example 6 and Comparative Example 5 show similar results when the core layer of varying thickness is flexed about a radius of 1 m. In Figures 14 (b), (c) and (d) the terms “Uwldxm” and “DGxm” have the corresponding meaning as described above for a radius of curvature of x m.

[144] Various aspects of the present invention are disclosed in the following clauses: Clause 1 - A core material for a sandwich panel, the core material comprising a core layer of expanded cellular foam composed of a thermoplastic polymer, the core layer having opposite first and second surfaces and a thickness of the core layer which extends between the first and second surfaces, wherein the core layer is divided into a series of segments by a plurality of planar dividing regions extending inwardly from the second surface in a thickness direction, the dividing regions being oriented transversely to a longitudinal direction of the core layer which extends between opposite first and second ends of the core layer, wherein adjacent segments are separated by a respective dividing region at which planar opposing side surfaces of the adjacent segments are in mutual contact, wherein at least the first surface, and the opposing side surfaces of the adjacent segments at each dividing region, comprise a non-cellular surface layer of the thermoplastic polymer which at least partially seals the expanded cellular foam inwardly of the respective surface layer from liquid ingress through the surface layer into the expanded cellular foam, and wherein each dividing region terminates at a closed end remote from the second surface, and the non-cellular surface layer at the first surface extends between the first surface and the closed end to form a flexible hinge between adjacent segments, whereby the core layer may be curved about an axis transverse to the longitudinal direction by flexing the core layer about one or more flexible hinges thereby hingedly to separate adjacent segments along respective dividing regions. Clause 2 - A core material according to clause 1 wherein the flexible hinge has a thickness between the closed end and the first surface of from 0.1 to 3 mm. Clause 3 - A core material according to clause 2 wherein the flexible hinge has a thickness between the closed end and the first surface of from 0.25 to 1 mm. Clause 4 - A core material according to any one of clauses 1 to 3 wherein the flexible hinge has a width, extending parallel to the distance between the planar opposing side surfaces, of from 100 to 3000 microns. Clause 5 - A core material according to clause 2 wherein the width of the flexible hinge is from 200 to 500 microns. Clause 6 - A core material according to any one of clauses 1 to 5 wherein the flexible hinge consists of the non-cellular surface layer at the first surface. Clause 7 - A core material according to any one of clauses 1 to 6 wherein the second surface also comprises a further non-cellular surface layer of the thermoplastic polymer which does not interconnect adjacent segments at the respective dividing regions. Clause 8 - A core material according to any one of clauses 1 to 7 wherein at each dividing region the opposing side surfaces of the adjacent segments which are in mutual contact are free of any bonding therebetween. Clause 9 - A core material according to any one of clauses I to 7 wherein at each dividing region the opposing side surfaces of the adjacent segments which are in mutual contact are weakly bonded together by a fusion-bonded connection therebetween, wherein the fusion-bonded connection is breakable by flexing the adjacent segments about the respective flexible hinge. Clause 10 - A core material according to clause 9 wherein the fusion-bonded connection comprises a plurality of individual, and mutually spaced, bond areas distributed across the dividing region. Clause 11 - A core material according to clause 10 wherein the bond areas are randomly distributed across the dividing region. Clause 12 - A core material according to any one of clauses 9 to 11 wherein the fusion-bonded connection has a tensile strength of from 0.05 to 4.0 MPa. Clause 13 - A core material according to clause 12 wherein the fusion-bonded connection has a tensile strength of from 0.07 to 0.12 MPa. Clause 14 - A core material according to any one of clauses 1 to 13 wherein at each dividing region the non-cellular surface layer, in the opposing side surfaces of the adjacent segments, has a depth of from 0.05 to 1.5 mm. Clause 15 - A core material according to clause 14 wherein at each dividing region the non-cellular surface layer, in the opposing side surfaces of the adjacent segments, has a depth of from 0.1 to 0.3 mm. Clause 16 - A core material according to any one of clauses 1 to 15 wherein at each dividing region the non-cellular surface layer, in the opposing side surfaces of the adjacent segments, has a gloss value of the surface measured at 60° in accordance with DIN 67530-1982 of from 1 to 10 gloss units. Clause 17 - A core material according to clause 16 wherein at each dividing region the non-cellular surface layer, in the opposing side surfaces of the adjacent segments, has a gloss value of the surface measured at 60° in accordance with 67530-1982 of from 4 to 7 gloss units. Clause 18 - A core material according to any one of clauses 1 to 17 wherein at the first surface the non-cellular surface layer has a depth of from 0.05 to 1.5 mm. Clause 19 - A core material according to clause 18 wherein at the first surface the non-cellular surface layer has a depth of from 0.1 to 0.3 mm. Clause 20 - A core material according to any one of clauses 1 to 19 wherein at the first surface the non-cellular surface layer has a gloss value of the surface measured at 60° in accordance with DIN 67530-1982 of from 1 to 10 gloss units. Clause 21 - A core material according to clause 20 wherein at the first surface the non-cellular surface layer has a gloss value of the surface measured at 60° in accordance with DIN 67530-1982 of from 4 to 7 gloss units. Clause 22 - A core material according to any one of clauses 1 to 21 wherein the thermoplastic polymer comprises a polyester, optionally a polyalkylene terephthalate, further optionally a polyethylene terephthalate. Clause 23 - A core material according to any one of clauses 1 to 22 wherein the thermoplastic polymer comprises polyethylene terephthalate and the expanded cellular foam has a density of from 60 to 300 kg / m3. Clause 24 - A core material according to any one of clauses 1 to 23 wherein the core layer has a thickness of from 5 to 80 mm. Clause 25 - A core material according to any one of clauses 1 to 24 wherein the dividing regions are parallel, and / or orthogonal to the longitudinal direction of the core layer. Clause 26 - A method of manufacturing a core material for a sandwich panel, the method comprising the steps of: a. providing a plurality of segments of expanded cellular foam composed of a thermoplastic polymer, the segments being assembled to form a layer comprising a series of the segments, in which adjacent segments are in mutual contact by respective opposite planar side surfaces which extend through the thickness of the layer, the thickness extending between opposite first and second surfaces of the layer, the side surfaces being transverse to a longitudinal direction of the layer which extends between opposite first and second ends of the layer, wherein the planar side surfaces of the adjacent segments which are in contact each comprise a non-cellular surface layer of the thermoplastic polymer; and b. applying heat to the first surface to heat the thermoplastic polymer and deforming the heated thermoplastic polymer to form a non-cellular surface layer of the thermoplastic polymer which extends inwardly of the first surface and forms a flexible hinge between adjacent segments, whereby the segments are interconnected at the first surface by a series of the flexible hinges to form a core layer which may be curved about an axis transverse to the longitudinal direction by flexing the core layer about one or more flexible hinges thereby hingedly to separate adjacent segments. Clause 27 - A method according to clause 26 wherein in step (b) the first surface of the layer is directly contacted by a heated surface of a heating member. Clause 28 - A method according to clause 27 wherein the heating member is a plate and the heated surface is planar or the heating member is a roller and the heated surface is cylindrical. Clause 29 - A method according to clause 27 or clause 28 wherein in step (b) the heated surface is at temperature which is within a range which extends from a lower value, which is the melting temperature of the thermoplastic polymer, to an upper value, which is up to 30 °C above the melting temperature of the thermoplastic polymer. Clause 30 - A method according to clause 29 wherein the thermoplastic polymer comprises polyethylene terephthalate and the heated surface is at temperature which is within the range of from 230 to 280 °C. Clause 31 - A method according to any one of clauses 27 to 30 wherein in step (b) the first surface of the layer is directly contacted by the heated surface of the heating member for a period of from 3 to 40 seconds. Clause 32 - A method according to clause 31 wherein in step (b) the first surface of the layer is directly contacted by the heated surface of the heating member for a period of from 15 to 20 seconds. Clause 33 - A method according to any one of clauses 27 to 32 wherein in step (b) the first surface of the layer is directly contacted by the heated surface of the heating member at an applied contact pressure of from 0.001 to 0.85 MPa. Clause 34 - A method according to clause 26 wherein in step (b) the first surface is heated by infra-red radiation to melt the first surface to form a surface layer of molten thermoplastic polymer and the molten thermoplastic polymer is deformed to form a continuous surface film of molten thermoplastic polymer. Clause 35 - A method according to clause 34 wherein in step (b) the first surface is moved relative to an infra-red lamp which heats the first surface by infra-red radiation, and relative to a scraper element which is positioned downstream of the infra-red lamp, in a direction of relative motion of the first surface, the scraper element having a scraper surface which extends across a transverse width of the first surface, and is configured to engage the heated thermoplastic polymer to deform the molten thermoplastic polymer at the surface and form the continuous surface film of molten thermoplastic polymer. Clause 36 - A method according to clause 35 wherein the scraper surface is urged against the first surface by the action of a roller which is aligned with, or positioned downstream of, the scraper element, the roller applying a line force against the second surface of the layer. Clause 37 - A method according to clause 36 wherein the line force, which is expressed as the force applied per unit width of the layer, the roller extending across a width direction of the layer, is within the range of from 10 to 80 N / m, optionally from 50 to 70 N / m, further optionally about 60 N / m. Clause 38 - A method according to any one of clauses 34 to 37 wherein in step (b) the infrared radiation is applied to the first surface at a power density of from 37.5 to 150 kW / m2, optionally from 37.5 to 150 kW / m2. Clause 39 - A method according to clause 38 wherein in step (b) the layer is translationally moved relative to the infra-red radiation at a speed of from 0.7 to 3 m / min, optionally from 1 to 1.5 m / min. Clause 40 - A method according to any one of clauses 34 to 39 wherein in step (b) the first surface of the layer is directly contacted by the scraper surface of the scraper element at an applied contact pressure of from 0.001 to 6.7 MPa. Clause 41 - A method according to any one of clauses 26 to 40 wherein the flexible hinge has a thickness extending inwardly from the first surface of from 0.1 to 3 mm. Clause 42 - A method according to clause 41 wherein the flexible hinge has a thickness extending inwardly from the first surface of from 0.25 to 1 mm. Clause 43 - A method according to any one of clauses 26 to 42 wherein the flexible hinge consists of the non-cellular surface layer at the first surface. Clause 44 - A method according to any one of clauses 26 to 43 wherein in step (b), simultaneously or sequentially with respect to the first surface, the second surface is heated to heat the thermoplastic polymer and the heated thermoplastic polymer is deformed to form a non-cellular surface layer of the thermoplastic polymer which extends inwardly of the second surface. Clause 45 - A method according to clause 44 wherein in step (b) the second surface of the layer is directly contacted by a heated surface of a second heating member. Clause 46 - A method according to clause 45 wherein the second heating member is a plate and the heated surface is planar or the second heating member is a roller and the heated surface is cylindrical. Clause 47 - A method according to clause 45 or clause 46 wherein in step (b) the heated surface of the second heating member is at temperature which is within a range which extends from a lower value, which is the melting temperature of the thermoplastic polymer, to an upper value, which is up to 30 °C above the melting temperature of the thermoplastic polymer. Clause 48 - A method according to clause 47 wherein the thermoplastic polymer comprises polyethylene terephthalate and the heated surface of the second member is at temperature which is within the range of from 230 to 280 °C. Clause 48 - A method according to any one of clauses 45 to 48 wherein in step (b) the second surface of the layer is directly contacted by the heated surface of the second heating member for a period of from 0.1 to 20 seconds. Clause 50 - A method according to clause 49 wherein in step (b) the second surface of the layer is directly contacted by the heated surface of the second heating member for a period of from 0.3 to 0.6 seconds. Clause 51 - A method according to any one of clauses 45 to 50 wherein in step (b) the second surface of the layer is directly contacted by the heated surface of the second heating member at an applied contact pressure of from 0.001 to 0.01 MPa. Clause 52 - A method according to clause 44 wherein in step (b) the second surface of the layer is heated by infra-red radiation to melt the second surface of the layer to form a surface layer of molten thermoplastic polymer, and the molten thermoplastic polymer is deformed to form a continuous surface film of molten thermoplastic polymer. Clause 53 - A method according to clause 52 wherein the second surface is moved relative to an infra-red lamp which heats the second surface by infra-red radiation, and relative to a scraper element which is positioned downstream of the infra-red lamp, in a direction of relative motion of the second surface, the scraper element having a scraper surface which extends across a transverse width of the second surface, and is configured engage the heated thermoplastic polymer to deform the molten thermoplastic polymer at the surface and form the continuous surface film of molten thermoplastic polymer. Clause 54 - A method according to any one of clauses 26 to 53 wherein the plurality of segments assembled to form the layer provided in step (a) is formed by the following steps, comprising: i.providing a plurality of a panels of the expanded cellular foam composed of thermoplastic polymer, each panel having opposite planar surfaces; ii.heating the thermoplastic polymer on each planar surface to form a surface layer of molten thermoplastic polymer and deforming the molten thermoplastic polymer to form a non-cellular surface layer of the thermoplastic polymer on each planar surface, the opposite planar surfaces being heated and deformed simultaneously or sequentially; iii.assembling the panels together to provide a block of the panels in which adjacent panels of the block comprise opposed facing surfaces, each facing surface comprising a respective non-cellular surface layer of the thermoplastic polymer; and iv.cutting through the block of panels to form the layer comprising the series of the segments, wherein each segment is formed from a cut portion of one of the respective panels. Clause 55 - A method according to any one of clauses 26 to 53 wherein the plurality of segments assembled to form the layer provided in step (a) is formed by the following steps, comprising: v.providing a panel of the expanded cellular foam composed of thermoplastic polymer, the panel having opposite planar surfaces; vi.heating the thermoplastic polymer on each planar surface to form a surface layer of molten thermoplastic polymer and deforming the molten thermoplastic polymer to form a non-cellular surface layer of the thermoplastic polymer on each planar surface, the opposite planar surfaces being heated and deformed simultaneously or sequentially; vii.cutting the panel to form a plurality of strips, each strip having opposite non-cellular surface layers of the thermoplastic polymer; and viii.assembling the strips together to provide the layer comprising the series of segments, wherein each segment is formed from a respective cut strip, and wherein adjacent segments of the layer comprise opposed facing surfaces, each facing surface comprising a respective non-cellular surface layer of the thermoplastic polymer. Clause 56 - A method according to clause 54 wherein in step (ii) pairs of the panels, as first and second panels, are disposed on opposite sides of a heating device so that each of the first and second panels has one of the planar surfaces thereof facing a respective heated surface of the heating device, and applying heat from the heating device to the facing surfaces of the first and second panels to heat the thermoplastic polymer and deforming the heated thermoplastic polymer to form a non-cellular surface layer of the thermoplastic polymer. Clause 57 - A method according to clause 56 wherein in step (ii) the facing surfaces of the first and second panels are directly contacted by a respective heated surface of the heating device. Clause 58 - A method according to clause 57 wherein the heating member is a heated metal plate and the heated surfaces are planar. Clause 59 - A method according to clause 57 or clause 58 wherein in step (ii) the heated surface is at temperature which is within a range which extends from a lower value, which is the melting temperature of the thermoplastic polymer, to an upper value, which is up to 30 °C above the melting temperature of the thermoplastic polymer. Clause 60 - A method according to clause 59 wherein the thermoplastic polymer comprises polyethylene terephthalate and the heated surface is at temperature which is within the range of from 230 to 280 °C. Clause 61 - A method according to any one of clauses 57 to 60 wherein in step (ii) the facing surfaces of the first and second panels are directly contacted by a respective heated surface of the heating device for a period of from 5 to 40 seconds. Clause 62 - A method according to clause 61 wherein in step (ii) the facing surfaces of the first and second panels are directly contacted by a respective heated surface of the heating device for a period of from 15 to 20 seconds. Clause 63 - A method according to any one of clauses 57 to 62 wherein in step (ii) the facing surfaces of the first and second panels are directly contacted by a respective heated surface of the heating device at an applied contact pressure of from O.OOlto 0.85 MPa. Clause 64 - A method according to clause 54 or clause 55 wherein in step (ii) each planar surface is heated by infra-red radiation to form a surface layer of molten thermoplastic polymer, and the molten thermoplastic polymer is deformed to form a continuous surface film of molten thermoplastic polymer. Clause 65 - A method according to clause 64 wherein in step (ii) the planar surface is moved relative to an infra-red lamp which heats the planar surface by infra-red radiation, and relative to a scraper element which is positioned downstream of the infra-red lamp, in a direction of relative motion of the planar surface, the scraper element having a scraper surface which extends across a transverse width of the planar surface, and is configured engage the heated thermoplastic polymer to deform the molten thermoplastic polymer at the surface and form the continuous surface film of molten thermoplastic polymer. Clause 66 - A method according to clause 54 or any clause dependent thereon wherein in step (iii) the non-cellular surface layers on the facing surfaces of the first and second panels are pressed together, and steps (ii) and (iii) are repeated on additional panels of the plurality of panels to successively form a block of the panels. Clause 67 - A method according to clause 66 wherein between steps (ii) and (iii) the heated facing surfaces of the first and second panels are subjected to a dwell step (x) in which the heated facing surfaces are permitted to cool prior to pressing the heated facing surfaces together in step (iii). Clause 68 - A method according to clause 67 wherein dwell step (x) is carried out for a period of from 5 to 15 seconds, and wherein during step (iii) the heated facing surfaces are bonded together by a fusion-bonded connection therebetween. Clause 69 - A method according to clause 68 wherein dwell step (x) is carried out for a period of from 5 to 10 seconds. Clause 70 - A method according to clause 68 or clause 69 wherein after step (iii) and subsequent cooling to ambient temperature, the opposed facing surfaces are weakly bonded together by the fusion-bonded connection therebetween, wherein the fusion-bonded connection is broken by flexing the adjacent segments about the respective flexible hinge. Clause 71 - A method according to clause 70 wherein the fusion-bonded connection comprises a plurality of individual, and mutually spaced, bond areas distributed across the facing surfaces. Clause 72 - A method according to clause 71 wherein the bond areas are randomly distributed across the facing surfaces. Clause 73 - A method according to any one of clauses 68 to 72 wherein the fusion-bonded connection has a tensile strength of from 0.05 to 4.0 MPa. Clause 74 - A method according to clause 73 wherein the fusion-bonded connection has a tensile strength of from 0.07 to 0.12 MPa. Clause 75 - A method according to clause 67 wherein dwell step (x) is carried out for period sufficient to cool the heated facing surfaces to solidify the non-cellular surface layers of the thermoplastic polymer prior to step (iii), whereby after step (iii) and subsequent cooling to ambient temperature, the opposed facing surfaces of the adjacent segments which are in contact are free of any bonding therebetween. Clause 76 - A method according to clause 75 wherein dwell step (x) is carried out for a period of from greater than 15 seconds.

[145] Various modifications to the disclosed embodiments will be apparent to those skilled in the art, and such modifications fall within the scope of the present invention as defined in the appended claims.

Claims

1. A core material for a sandwich panel, the core material comprising a core layer of expanded cellular foam composed of a thermoplastic polymer, the core layer having opposite first and second surfaces and a thickness of the core layer which extends between the first and second surfaces, wherein the core layer is divided into a series of segments by a plurality of planar dividing regions extending inwardly from the second surface in a thickness direction, the dividing regions being oriented transversely to a longitudinal direction of the core layer which extends between opposite first and second ends of the core layer, wherein adjacent segments are separated by a respective dividing region at which planar opposing side surfaces of the adjacent segments are in mutual contact, wherein at least the first surface, and the opposing side surfaces of the adjacent segments at each dividing region, comprise a non-cellular surface layer of the thermoplastic polymer which at least partially seals the expanded cellular foam inwardly of the respective surface layer from liquid ingress through the surface layer into the expanded cellular foam, and wherein each dividing region terminates at a closed end remote from the second surface, and the non-cellular surface layer at the first surface extends between the first surface and the closed end to form a flexible hinge between adjacent segments, whereby the core layer may be curved about an axis transverse to the longitudinal direction by flexing the core layer about one or more flexible hinges thereby hingedly to separate adjacent segments along respective dividing regions.

2. A core material according to claim 1 wherein the flexible hinge has a thickness between the closed end and the first surface of from 0.1 to 3 mm or from 0.25 to 1 mm.

3. A core material according to claim 1 or claim 2 wherein the flexible hinge has a width, extending parallel to the distance between the planar opposing side surfaces, of from 100 to 3000 microns or from 200 to 500 microns.

4. A core material according to any one of claims 1 to 3 wherein the flexible hinge consists of the non-cellular surface layer at the first surface.

5. A core material according to any one of claims 1 to 4 wherein at each dividing region the opposing side surfaces of the adjacent segments which are in mutual contact are free of any bonding therebetween.

6. A core material according to any one of claims 1 to 4 wherein at each dividing region the opposing side surfaces of the adjacent segments which are in mutual contact are weakly bonded together by a fusion-bonded connection therebetween, wherein the fusion-bonded connection is breakable by flexing the adjacent segments about the respective flexible hinge.

7. A core material according to claim 6 wherein the fusion-bonded connection comprises a plurality of individual, and mutually spaced, bond areas distributed across the dividing region, optionally wherein the bond areas are randomly distributed across the dividing region.

8. A core material according to claim 6 or claim 7 wherein the fusion-bonded connection has a tensile strength of from 0.05 to 4.0 MPa or from 0.07 to 0.12 MPa.

9. A core material according to any one of claims 1 to 8 wherein at each dividing region the non-cellular surface layer, in the opposing side surfaces of the adjacent segments, has a depth of from 0.05 to 1.5 mm or from 0.1 to 0.3 mm.

10. A core material according to any one of claims 1 to 9 wherein at each dividing region the non-cellular surface layer, in the opposing side surfaces of the adjacent segments, has a gloss value of the surface measured at 60° in accordance with DIN 67530-1982 of from 1 to 10 gloss units, or from 4 to 7 gloss units.

11. A core material according to any one of claims 1 to 10 wherein at the first surface the non-cellular surface layer has a depth of from 0.05 to 1.5 mm or from 0.1 to 0.3 mm.

12. A core material according to any one of claims 1 to 11 wherein at the first surface the non-cellular surface layer has a gloss value of the surface measured at 60° in accordance with DIN 67530-1982 of from 1 to 10 gloss units or from 4 to 7 gloss units.

13. A method of manufacturing a core material for a sandwich panel, the method comprising the steps of:a. providing a plurality of segments of expanded cellular foam composed of a thermoplastic polymer, the segments being assembled to form a layer comprising a series of the segments, in which adjacent segments are in mutual contact by respective opposite planar side surfaces which extend through the thickness of the layer, the thickness extending between opposite first and second surfaces of the layer, the side surfaces being transverse to a longitudinal direction of the layer which extends between opposite first and second ends of the layer, wherein the planar side surfaces of the adjacent segments which are in contact each comprise a non-cellular surface layer of the thermoplastic polymer; andb. applying heat to the first surface to heat the thermoplastic polymer and deforming the heated thermoplastic polymer to form a non-cellular surface layer of the thermoplastic polymer which extends inwardly of the first surface and forms a flexible hinge between adjacent segments, whereby the segments are interconnected at the first surface by a series of the flexible hinges to form a core layer which may be curved about an axis transverse to the longitudinal direction by flexing the core layer about one or more flexible hinges thereby hingedly to separate adjacent segments.

14. A method according to claim 13 wherein in step (b) the first surface of the layer is directly contacted by a heated surface of a heating member.

15. A method according to claim 14 wherein in step (b) the heated surface is at temperature which is within a range which extends from a lower value, which is the melting temperature of the thermoplastic polymer, to an upper value, which is up to 30 °C above the melting temperature of the thermoplastic polymer, optionally wherein the thermoplastic polymer comprises polyethylene terephthalate and the heated surface is at temperature which is within the range of from 230 to 280 °C.

16. A method according to claim 14 or claim 15 wherein in step (b) the first surface of the layer is directly contacted by the heated surface of the heating member for a period of from 3 to 40 seconds or from 15 to 20 seconds, and / or at an applied contact pressure of from 0.001 to 0.85 MPa.

17. A method according to claim 13 wherein in step (b) the first surface is heated by infrared radiation to melt the first surface to form a surface layer of molten thermoplastic polymer and the molten thermoplastic polymer is deformed to form a continuous surface film of molten thermoplastic polymer.

18. A method according to claim 17 wherein in step (b) the first surface is moved relative to an infra-red lamp which heats the first surface by infra-red radiation, and relative to a scraper element which is positioned downstream of the infra-red lamp, in a direction of relative motion of the first surface, the scraper element having a scraper surface which extends across a transverse width of the first surface, and is configured to engage the heated thermoplastic polymer to deform the molten thermoplastic polymer at the surface and form the continuous surface film of molten thermoplastic polymer.

19. A method according to claim 18 wherein the scraper surface is urged against the first surface by the action of a roller which is aligned with, or positioned downstream of, the scraper element, the roller applying a line force against the second surface of the layer, optionally wherein the line force, which is expressed as the force applied per unit width of the layer, the roller extending across a width direction of the layer, is within the range of from 10 to 80 N / m, or from 50 to 70 N / m, or about 60 N / m.

20. A method according to any one of claims 17 to 19 wherein in step (b) the infra-red radiation is applied to the first surface at a power density of from 37.5 to 150 kW / m2, optionally from 37.5 to 150 kW / m2.

21. A method according to claim 20 wherein in step (b) the layer is translationally moved relative to the infra-red radiation at a speed of from 0.7 to 3 m / min, optionally from 1 to 1.5 m / min.

22. A method according to any one of claims 17 to 21 wherein in step (b) the first surface of the layer is directly contacted by the scraper surface of the scraper element at an applied contact pressure of from 0.001 to 6.7 MPa.

23. A method according to any one of claims 13 to 22 wherein the flexible hinge has a thickness extending inwardly from the first surface of from 0.1 to 3 mm or from 0.25 to 1 mm.

24. A method according to any one of claims 13 to 23 wherein the flexible hinge consists of the non-cellular surface layer at the first surface.

25. A method according to any one of claims 13 to 24 wherein the plurality of segments assembled to form the layer provided in step (a) is formed by the following steps, comprising:i. providing a plurality of a panels of the expanded cellular foam composed of thermoplastic polymer, each panel having opposite planar surfaces;ii. heating the thermoplastic polymer on each planar surface to form a surface layer of molten thermoplastic polymer and deforming the molten thermoplastic polymer to form a non-cellular surface layer of the thermoplastic polymer on each planar surface, the opposite planar surfaces being heated and deformed simultaneously or sequentially;iii. assembling the panels together to provide a block of the panels in which adjacent panels of the block comprise opposed facing surfaces, each facing surface comprising a respective non-cellular surface layer of the thermoplastic polymer; andiv. cutting through the block of panels to form the layer comprising the series of the segments, wherein each segment is formed from a cut portion of one of the respective panels.

26. A method according to any one of claims 13 to 24 wherein the plurality of segments assembled to form the layer provided in step (a) is formed by the following steps, comprising:i. providing a panel of the expanded cellular foam composed of thermoplastic polymer, the panel having opposite planar surfaces;ii. heating the thermoplastic polymer on each planar surface to form a surface layer of molten thermoplastic polymer and deforming the molten thermoplastic polymer to form a non-cellular surface layer of the thermoplastic polymer on each planar surface, the opposite planar surfaces being heated and deformed simultaneously or sequentially;iii. cutting the panel to form a plurality of strips, each strip having opposite non-cellular surface layers of the thermoplastic polymer; andiv. assembling the strips together to provide the layer comprising the series of segments, wherein each segment is formed from a respective cut strip, and wherein adjacent segments of the layer comprise opposed facing surfaces, each facing surface comprising a respective non-cellular surface layer of the thermoplastic polymer.

27. A method according to claim 25 wherein in step (ii) pairs of the panels, as first and second panels, are disposed on opposite sides of a heating device so that each of the first and second panels has one of the planar surfaces thereof facing a respective heated surface of the heating device, and applying heat from the heating device to the facing surfaces of the first and second panels to heat the thermoplastic polymer and deforming the heated thermoplastic polymer to form a non-cellular surface layer of the thermoplastic polymer.

28. A method according to claim 25 or claim 26 wherein in step (ii) each planar surface is heated by infra-red radiation to form a surface layer of molten thermoplastic polymer, and the molten thermoplastic polymer is deformed to form a continuous surface film of molten thermoplastic polymer.

29. A method according to claim 28 wherein in step (ii) the planar surface is moved relative to an infra-red lamp which heats the planar surface by infra-red radiation, and relative to a scraper element which is positioned downstream of the infra-red lamp, in a direction of relative motion of the planar surface, the scraper element having a scraper surface which extends across a transverse width of the planar surface, and is configured engage the heated thermoplastic polymer to deform the molten thermoplastic polymer at the surface and form the continuous surface film of molten thermoplastic polymer.

30. A method according to claim 25 or any claim dependent thereon wherein in step (iii) the non-cellular surface layers on the facing surfaces of the first and second panels are pressed together, and steps (ii) and (iii) are repeated on additional panels of the plurality of panels to successively form a block of the panels.

31. A method according to claim 30 wherein between steps (ii) and (iii) the heated facing surfaces of the first and second panels are subjected to a dwell step (x) in which the heated facing surfaces are permitted to cool prior to pressing the heated facing surfaces together in step (iii).

32. A method according to claim 31 wherein dwell step (x) is carried out for a period of from 5 to 15 seconds or 5 to 10 seconds, and wherein during step (iii) the heated facing surfaces are bonded together by a fusion-bonded connection therebetween.

33. A method according to claim 32 wherein after step (iii) and subsequent cooling to ambient temperature, the opposed facing surfaces are weakly bonded together by the fusion-bonded connection therebetween, wherein the fusion-bonded connection is broken by flexing the adjacent segments about the respective flexible hinge, optionally wherein the fusion-bonded connection comprises a plurality of individual, and mutually spaced, bond areas distributed, typically randomly, across the facing surfaces.

34. A method according to claim 31 wherein dwell step (x) is carried out for period sufficient to cool the heated facing surfaces to solidify the non-cellular surface layers of the thermoplastic polymer prior to step (iii), whereby after step (iii) and subsequent cooling to ambient temperature, the opposed facing surfaces of the adjacent segments which are in contact are free of any bonding therebetween.

35. A method according to claim 34 wherein dwell step (x) is carried out for a period of from greater than 15 seconds.42

Images