GB2619346A

GB2619346A - Composite Sheet and Parts

Info

Publication number: GB2619346A
Application number: GB2208139.2A
Authority: GB
Inventors: Thompson Rob; Siddall Ben; Rodriguez Jorge
Original assignee: Arrival UK Ltd
Current assignee: Arrival UK Ltd
Priority date: 2022-06-01
Filing date: 2022-06-01
Publication date: 2023-12-06
Also published as: GB202208139D0

Abstract

A sheet comprises an adhesive layer 106 between a thermoplastic layer 104 (preferably 0.1-5 mm thick) and a hard coat layer 102 (preferably 5-1000 µm thick). In one composite sheet the thermoplastic is polypropylene (PP) and the sheet may be thermoformed at the PP forming temperature. Preferably, the heat activated (e.g., hot melt) adhesive is coated onto the scratch resistant hardcoat and laminated to the PP. Another composite sheet includes a fibre reinforced thermoplastic (202, figure 2) and has the hard-coat providing a surface finish. The composite sheet is used as a vehicle part. The sheet may also include a colour layer 108 and metal layer 110. Disclosed fibre reinforced thermoplastic includes interwoven glass and polypropylene fibres. The thermoplastic may be extruded (404, figure 4) and then laminated using calender rollers (410, figure 4). Disclosed hard-coats include polyurethane (PU), polymethyl methacrylate (PMMA) and polyvinylidene fluoride (PVDF). Disclosed adhesives comprise PU, PP, polyamdie, polyester or crosslinking adhesive.

Description

Composite Sheet and Parts The present disclosure relates to a composite sheet for forming a surface layer in a thermoplastic composite structure, and a composite structure comprising the composite sheet. The disclosure also relates to methods for manufacturing a composite sheet and structure.

Composite panels and parts formed from thermoplastic polymers comprising reinforcing fibres are desirable for their light weight, high strength and versatility. However, certain thermoplastic polymers have a dull surface finish and are prone to being scratched easily. This currently prohibits the use of such composites on the surface of panels and parts that must be aesthetically pleasing, i.e. have a smooth and glossy surface finish, or requires additional finishing treatments to achieve the desired finish.

Aspects are set out in the independent claims and optional features are set out in the claims dependent thereto.

In some aspects, there is provided a composite sheet for forming a surface layer in a thermoplastic composite structure, the composite sheet comprising a thermoplastic layer; a hardcoat layer; and an adhesive layer between the thermoplastic layer and the hardcoat layer, wherein the composite sheet is thermoformable at a forming temperature of the thermoplastic layer. A sheet may be defined as a film or other broad continuous surface that is substantially longer or wider than it is thin. The thermoplastic layer may be an extruded polymer sheet and/or may comprise polypropylene or polyester, for example. The term "thermoformable" means that the composite sheet becomes pliable or mouldable at a forming temperature, such that it can be formed into a specific shape. The forming temperature of a material is usually less than its melting point, meaning the composite sheet is also thermoformable at the melting point of the thermoplastic layer. Crucially, even if the composite sheet is not heated to a forming temperature of the hardcoat layer or adhesive layer (i.e. the forming temperature of the material of the hardcoat or adhesive layer in bulk form), the sheet can still be thermoformed. This may be, for example, due to the low thickness and/or rigidity of the hardcoat and/or adhesive layers. Thus, it is the forming temperature of the thermoplastic layer that dictates whether or not the composite sheet is thermoformable. As such, the hardcoat and adhesive layers may not necessarily undergo any physical transition on a molecular level when the sheet is thermoformed, and it is only the thermoplastic layer which is truly thermoformed (i.e. becomes substantially more pliable at the forming temperature) and which provides the sheet with its structural integrity before and after thermoforming.

The hardcoat layer (which may also be known as a clearcoat, paintfilm or cap) protects the thermoplastic layer, is scratch resistant, and can have a glossy surface. The hardcoat layer may therefore have a greater scratch resistance than the thermoplastic layer. That is to say that the hardcoat may be harder than the thermoplastic layer (e.g. have a greater Vickers, Brinell, Rockwell, Meyer, Mohs and/or Leeb hardness). Additionally or alternatively, it may have a lower surface friction, which also improves scratch resistance. The hardcoat may be polyvinylidene difluoride (PVDF), poly(methyl methacrylate) (PMMA) polyurethane (PU) or any other suitable polymer, with scratch resistant polymers being preferred in general, and the polymer may or may not be crosslinked. The adhesive layer, which may also be known as a primer layer, may comprise a heat activated adhesive or UV activated adhesive which may or may not have already been activated or cured. The adhesive may be a hotmelt adhesive such as a thermoplastic hotmelt adhesive such as a polyurethane, polypropylene, polyamide, polyester hotmelt adhesive. Alternatively, the adhesive may be heat activated crosslinking adhesive. The forming temperature and/or melting point of the thermoplastic layer may be less than the forming temperature and/or melting point of the hardcoat and/or adhesive layers.

The thickness of the thermoplastic layer may be greater than the thickness of the hardcoat layer and/or adhesive layer. The thickness of the thermoplastic layer may be less than 5 mm, 2 mm, 1 mm or 0.5 mm. The thickness of the hardcoat layer and/or adhesive layer may be less than 1000 pm, 500 pm, 200 pm, 100 pm or 50 pm. The thinner the hardcoat, the more formable it is at the forming temperature of the thermoplastic layer, since it is less rigid and can take on the shape of a mould more readily. The thickness of the thermoplastic layer may be greater than 0.4 mm, 0.3 mm, 0.2 mm or 0.1 mm. The thicker the thermoplastic layer, the better it is at preventing print-through of structural fibres when incorporated into a composite structure or part. The thickness of the hardcoat layer and/or adhesive layer may be greater than 5 pm, 10 pm, 20 pm or 40 pm. The thicker the hardcoat layer, the greater the protection it provides when the composite sheet is incorporated into a composite structure or part.

The composite sheet can comprise additional layers under the hardcoat such as a metal layer, colour layer or print layer, which provide the sheet with a corresponding finish. The thermoplastic layer may comprise one or more of a pigment, a dye, a flame retardant and a UV-absorbing additive.

In some aspects, there is provided a composite structure comprising: a structural layer comprising reinforcing fibres (such as glass or carbon fibres) and a thermoplastic body polymer; and a surface layer comprising the composite sheet described above, wherein the hardcoat layer of the composite sheet provides a surface finish to the composite structure. The composite structure may be used to make a vehicle part or body panel using thermoforming. By providing a surface layer without the reinforcing fibres, the reinforcing fibres are shielded from the surface and print-through is consequently reduced, resulting in an improved surface finish, while the hardcoat also improves the durability of the surface. The term "composite structure" as used in this disclosure encompasses both a precursor structure (wherein the layers are not fused) for making a composite part, and a fused structure wherein the structural layer and thermoplastic layer are fused together, the latter also being a composite part. In some embodiments, the surface layer, when consolidated with the structural layer, is substantially free of fibres. That is to say that there are no reinforcing or other fibres in the surface layer.

The reinforcing fibres and thermoplastic body polymer may form a plurality of alternating fibre layers and thermoplastic body polymer layers. The reinforcing fibres may form a mesh, wherein the thermoplastic body polymer encompasses the mesh of reinforcing fibres. A mesh may be woven or non-woven fabric, or layers of such a fabric. A woven fabric may be woven from a yarn of combined reinforcing fibres and thermoplastic polymer fibres. These fibres can be commingled or otherwise combined with the reinforcing fibres to form a combined yarn, which can be woven into a fabric or mesh. The thermoplastic body polymer and/or thermoplastic layer of the surface layer/composite sheet may be a polypropylene-based, polyamide-based, polycarbonate-based or polyester-based thermoplastic polymer. The thermoplastic body polymer and the thermoplastic surface layer polymer may be the same or different polymers.

The term "yarn" is used in a broad sense to cover any yarn of a suitable form factor to be woven, knitted or otherwise constructed into a fabric. In the same sense, the term "fibre" is understood to cover a wide range of filaments and cross-sectional form factors of the fibre and is understood to cover tape, and in particular spread tow tape, in relation to either or both of reinforcing and thermoplastic polymer fibres.

In some aspects, there is provided a method of manufacturing the composite sheet described above. The method comprises: providing a film comprising an adhesive layer on a hardcoat layer; providing (e.g. by extruding) a thermoplastic sheet (e.g. polypropylene thermoplastic sheet); and laminating the thermoplastic sheet and the film (optionally using a calender, such as nip rolls), wherein the adhesive layer of the film contacts the thermoplastic sheet. The film may comprise additional layers, such as those described above in respect of the composite sheet. The film may be provided as a roll and be fed from the roll to be laminated (e.g. it can be fed from the roll to a calender). As would be understood by the skilled person, the calendar can be used to reduce or control the thickness of the thermoplastic sheet and total thickness of the final composite sheet. The final composite sheet may be fed onto a roll.

The adhesive layer may comprise a heat activated adhesive. Further, the temperature of the thermoplastic sheet may be above the activation temperature of the heat activated adhesive, such that when the adhesive and hardcoat film contact the thermoplastic, the adhesive cures and the film adheres to the thermoplastic. Laminating the hardcoat film onto the thermoplastic sheet during the extrusion process, when it is still hot from forming is particularly advantageous since 1) the thickness of the thermoplastic can be controlled, 2) the adhesive can be cured and 3) the film and thermoplastic sheet can be laminated all in a single step.

In some aspects, there is provided a method of manufacturing a part comprising a composite structure. The method comprises: preforming the composite sheet described above using a male mould to form a preformed composite sheet; arranging the preformed composite sheet in a female mould, wherein the female mould is complementary to the male mould; arranging a structural layer comprising reinforcing fibres and a thermoplastic body polymer on top of the preformed composite sheet; raising the temperature of the structural layer and the composite sheet above a reaction threshold temperature to fuse the structural layer and the composite sheet; cooling the structural layer and composite sheet to below the reaction threshold temperature to form the part; and removing the part from the mould. The method may further comprise applying a pressure differential, for example positive pressure applied from outside the mould, across the composite structure while the temperature of the composite structure is above the reaction threshold temperature.

In this context, preforming means heating the composite sheet above a forming temperature of the thermoplastic layer of the composite sheet and then cooling it down again. This conforms the sheet to the male mould such that it takes on the shape of the male mould. Since the male mould is complementary to the female mould (i.e. it is convex while the female mould is concave and the respective dimensions are substantially equal) after preforming, the composite sheet can be positioned in the female mould without folding, stretching, tearing or otherwise disrupting the sheet in a manner that could spoil the surface finish of the final part. As would be understood, the male and female moulds may not be perfectly complementary and may indeed have slightly differing external and internal dimensions to compensate for the thickness of the composite sheet.

The reaction threshold temperature may be a temperature threshold at which a physical reaction of the thermoplastic polymers (i.e. of the thermoplastic layer and body polymer) occurs. Examples of a physical reaction include sintering, melting or partially melting the thermoplastics polymers. A physical reaction may also comprise consolidating the thermoplastic body polymer and reinforcing fibres, for example heating them such that the thermoplastic body polymer becomes fluid enough to flow around the structural fibres. The reaction threshold temperature may be but is not limited to the melting point of the thermoplastic polymers or the glass transition temperature of the thermoplastic polymers of the composite structure. The reaction threshold temperature may be equal to or above the highest of the glass transition/melting temperature of the thermoplastic body polymer and the glass transition/melting temperature of the thermoplastic layer of the composite sheet, although it may also be lower, so long as the thermoplastic polymers are softened, for example such that the thermoplastic body polymer bonds together. The reaction threshold temperature is typically between 180-220 °C, depending of course on the nature of the thermoplastic polymers present. For example, this temperature range is suitable for polypropylene. The method may further comprise applying a pressure differential across the composite structure while the temperature of the composite structure is above the reaction threshold temperature.

The hardcoat layer may not directly contact the male mould, while it may directly contact the female mould. During the fusing process, the hardcoat can take on the smoothness and surface finish of the female mould, improving the surface finish by, for example, making it smoother and therefore glossier. The temperature may be raised above a melting point of the hardcoat to further encourage the hardcoat to take on the surface finish of the mould, although it has been found that the hardcoat does not necessarily need to melt for this to happen.

Embodiments will now be described by way of example with reference to the drawings of which: Figure 1 illustrates an exploded view of a composite sheet; Figure 2 illustrates an exploded view of an unconsolidated composite structure having a composite sheet surface layer on a structural layer; Figure 3 is a flow diagram for a method of manufacturing a composite sheet; Figure 4 illustrates a system for manufacturing a composite sheet; and Figure 5 is a flow diagram for a method of manufacturing a part comprising a composite structure.

With reference to Figure 1, in first embodiments, a composite sheet 100 comprises a thermoplastic layer 104 and a hardcoat layer 102. The thermoplastic layer 104 is adhered to the hardcoat layer 102 by an adhesive layer 106 between the thermoplastic layer 104 and the hardcoat layer 102. The thermoplastic layer 104 is a polypropylene sheet. The thermoplastic layer 104 is 500 pm thick, although other thicknesses may also be used. The hardcoat layer 102 is made from polyvinylidene difluoride and is 50 pm thick, although other polymers and thicknesses may also be used. The adhesive layer is polypropylene-compatible heat activated adhesive and is 15 pm thick, although any suitable adhesive and adhesive thickness may be used instead. The composite sheet in Figure 1 also comprises a colour layer 108 (50 pm thick) and a metal layer 110 (<0.3 pm thick) in order to provide the composite sheet 100 with a coloured metallic look, however these additional layers are optional. Figure 1 illustrates an exploded view of the composite sheet for ease of understanding.

With reference to Figure 2, in second embodiments a composite structure 200 comprises a composite sheet 100 of the first embodiments (without a colour layer 108 or metal layer 110 in this instance) on a structural layer 202. Figure 2 illustrates an exploded view of the composite structure for ease of understanding. The structural layer 202 comprises layers of reinforcing glass fibres and polypropylene fibres (referred to as a thermoplastic body polymer), although other suitable reinforcing fibres and thermoplastic polymer fibres are also envisaged. For example, the structural layer 202 may be made from a woven fabric, specifically a combined glass and thermoplastic weave, which comprises glass fibres commingled or otherwise combined with polypropylene filaments that form a combined yarn, with the combined yarn woven into a fabric.

Once the composite structure is consolidated (in the manner described later, for example), the polypropylene thermoplastic layer 104 of the composite sheet 100 fuses with the polypropylene fibres of the structural layer 202, which also fuse together and incorporate the glass fibres. Once fused together, the structural layer 202 provides the composite structure 200 with the majority of its structural strength. The hardcoat layer 102 provides the composite structure 200 with a scratch resistant surface finish.

The structural layer 202 may be formed itself from several sublayers, for example several layers of a weave or fabric as described above; other pre-cursor fabrics or weaves; or alternating layers of the thermoplastic body polymer and reinforcing fibres, for example alternating layers of a sheet of the thermoplastic body polymer and of a layer of a fabric or weave of reinforcing fibre. It will be appreciated that many combinations of such sub layers and alternative arrangements of sublayers forming the structural layer 202 are possible, without departing from the present disclosure. In some embodiments, the reinforcing fibres have a diameter in the range of 15-20 pm.

Figure 3 is a flow diagram for a method of manufacturing the composite sheet 100 described above. The method comprises providing 300 a film comprising the adhesive layer 106 on the hardcoat layer 102; extruding 302 the thermoplastic layer 104 as a thermoplastic sheet; and laminating 304 the thermoplastic sheet and the film using a calender, wherein the adhesive layer of the film contacts the thermoplastic sheet. Other methods of lamination known to the skilled person may be used instead, although the use of a calender is particularly advantageous. With reference to Figure 4, which illustrates a system 400 for manufacturing composite sheet 100, thermoplastic 402 is extruded from extruder 404. The hardcoat/adhesive film is provided as a roll 406. Both the film and thermoplastic 402 are fed into the calender 410. As would be understood by the skilled person, the calendar can also be used to reduce or control the thickness of the thermoplastic 402 and total thickness of the final composite sheet 100. The final composite sheet 100 is then fed onto a composite sheet roll 408.

The adhesive layer 106 comprises a heat activated adhesive, and the temperature of the extruded thermoplastic 402 is above the activation temperature of the heat activated adhesive, such that when the adhesive and hardcoat film contact the thermoplastic 402, the adhesive cures and the film adheres to the thermoplastic.

Figure 5 is a flow diagram for a method of manufacturing a part comprising the layers of the composite structure 200 described above. The method comprises: preforming 500 a composite sheet 100 (such as the composite sheet 100 illustrated in Figure 1) using a male mould to form a preformed composite sheet; arranging 502 the preformed composite sheet in a female mould, wherein the female mould is complementary to the male mould; arranging 504 a structural layer 202 comprising reinforcing fibres and a thermoplastic body polymer on top of the preformed composite sheet; raising 506 the temperature of the composite sheet 100 and the structural layer 202 above a reaction threshold temperature to fuse the structural layer 202 and composite sheet 100; cooling 508 the composite sheet 100 and the structural layer 202 to below the reaction threshold temperature to form the part; and removing 510 the part from the mould.

Preforming 500 the composite sheet 100 comprises heating the composite sheet 100 above 140 °C, at which point the polypropylene layer 104 of the composite sheet 100 softens enough for the composite sheet 100 to take on the shape of the mould. It also retains this shape after it has been cooled down again. As would be understood by the skilled person, the exact temperature required depends on the polymer used, and can be readily determined by raising the temperature until the composite sheet takes the shape of the mould. Since the hardcoat layer 102 is so thin, it does not prevent the composite sheet 100 from being thermoformed, irrespective of whether or not it softens significantly at the same temperature as the thermoplastic layer 104. The orientation of the composite sheet 100 is such that the hardcoat layer 102 does not directly contact the male mould.

Raising 506 the temperature of the composite sheet 100 and the structural layer 202 above a reaction threshold temperature also comprises simultaneously applying a pressure differential across the composite structure 200 while the temperature of the composite structure 200 is above the reaction threshold temperature, to encourage the layers to take the shape of the mould, however applying pressure is not essential. The pressure differential applied may be 1 atmosphere. The pressure differential may be applied by negative pressure, evacuating air from between the mould and the layers, or by positive pressure to the layers from outside the mould, or both. In the present embodiment where both the thermoplastic layer 104 and the thermoplastic body polymer fibres of the structural layer 202 are both polypropylene, a reaction threshold temperature of 200 °C is used. During manufacture of the composite structure, the layers are assembled and cut before being placed in the mould, although this is optional.

The hardcoat layer 102 directly contacts the female mould. Thus, during the consolidation process, the hardcoat 102 takes on the surface finish of the female mould, improving its own surface finish by making it smoother and glossier. Of course, if a glossy surface finish is not desired, the female mould need not have a smooth surface and can indeed have a rough or textured surface to deliberately apply a rough finish to the hardcoat 102. The temperature may be raised above a melting point of the hardcoat layer 102 to further encourage the hardcoat to take on the surface finish of the mould.

The softened or molten layers are subsequently cooled 508 to below the reaction threshold temperature, at which point the composite sheet 100 and structural layer 202 become fixed in the shape of the mould. It will be appreciated that in some embodiments the steps of softening or melting the layers and moulding of the softened or molten layers may proceed in parallel, that is the initially cool layers may be urged into contact with the mould and heated at the same or at a subsequent time while continuing to urge the composite structure 200 into contact with the mould.

It will be appreciated that the above description is made by way of example and not limitation of the scope of the appended claims, including any equivalents as included within the scope of the claims. Various modifications are possible and will be readily apparent to the skilled person in the art. Likewise, features of the described embodiments can be combined with any appropriate aspect described above and optional features of any one aspect can be combined with any other appropriate aspect.

Claims

Claims 1. A composite sheet for forming a surface layer in a thermoplastic composite structure, the composite sheet comprising: a polypropylene thermoplastic layer; a hardcoat layer; and an adhesive layer between the polypropylene thermoplastic layer and the hardcoat layer, wherein the composite sheet is thermoformable at a forming temperature of the polypropylene thermoplastic layer.
2. The composite sheet of claim 1, wherein the hardcoat layer has a greater scratch resistance than the polypropylene thermoplastic layer.
3. The composite sheet of any preceding claim, wherein the adhesive layer comprises a heat activated adhesive.
4. The composite sheet of any preceding claim, wherein the melting point of the polypropylene thermoplastic layer is less than the melting point of the hardcoat layer.
5. The composite sheet of any preceding claim, wherein the thickness of the polypropylene thermoplastic layer is greater than the thickness of the hardcoat layer and/or wherein the thickness of the polypropylene thermoplastic layer is less than 5 mm, 2 mm, 1 mm or 0.5 mm and/or the thickness of the hardcoat layer is less than 1000 pm, 500 pm, 200 pm, 100 pm or 50 pm.
6. The composite sheet of any preceding claim, wherein the thickness of the polypropylene thermoplastic layer is greater than 0.4 mm, 0.3 mm, 0.2 mm or 0.1 mm and/or the thickness of the hardcoat layer is greater than 5 pm, 10 pm, 20 pm or 40 pm.
7. A composite structure comprising: a structural layer comprising reinforcing fibres and a thermoplastic body polymer; and a surface layer comprising: a thermoplastic layer; a hardcoat layer; and an adhesive layer between the thermoplastic layer and the hardcoat layer, wherein the surface layer is thermoformable at a forming temperature of the thermoplastic layer, and wherein the hardcoat layer provides a surface finish to the composite structure.
8. The composite structure of claim 7, wherein the structural layer is formed from one or more layers of a mesh of a yarn of the reinforcing fibres combined with fibres of the thermoplastic body polymer.
9. The composite structure of claim 7 or 8, wherein the surface layer, when consolidated with the structural layer, is substantially free of reinforcing fibres.
10. The composite structure of claim 7, 8 or 9, wherein the thermoplastic layer and the thermoplastic body polymer both comprise the same polymer and/or wherein the structural layer and thermoplastic layer are fused together.
11. A vehicle part or body panel formed from the composite structure of any of claims 7 to 10.
12. A method of manufacturing the composite sheet of any of claims 1 to 6, the method comprising: providing a film comprising an adhesive layer on a hardcoat layer; providing a polypropylene thermoplastic; and laminating the polypropylene thermoplastic and the film, wherein the adhesive layer of the film contacts the thermoplastic.
13. The method of claim 12, wherein the adhesive layer comprises a heat activated adhesive, and the temperature of the polypropylene thermoplastic is above the activation temperature of the heat activated adhesive.
14. A method of manufacturing a part comprising a composite structure, the method comprising: preforming a composite sheet using a male mould to form a preformed composite sheet, wherein the composite sheet comprises: a thermoplastic layer; a hardcoat layer; and an adhesive layer between the thermoplastic layer and the hardcoat layer; arranging the preformed composite sheet in a female mould, wherein the female mould is complementary to the male mould; arranging a structural layer comprising reinforcing fibres and a thermoplastic body polymer on top of the preformed composite sheet; raising the temperature of the structural layer and the composite sheet above a reaction threshold temperature to fuse the structural layer and composite sheet; cooling the structural layer and composite sheet to below the reaction threshold temperature to form the part; and removing the part from the female mould.
15. The method of claim 14, 15 or 16, wherein the method further comprises applying a pressure differential across the composite structure while the temperature of the composite structure is above the reaction threshold temperature.